This patch adds a helper to determine if snapshots are external and uses
the helper to fix detection of those in snapshot deletion code.

Snapshots are external if they have an external memory image or if the
disk locations are external. As mixed snapshots are forbidden for now
we need to check just one disk to know.

Currently, if user calls virDomainAbortJob we just issue
'migrate_cancel' and hope for the best. However, if user calls
the API in wrong phase when migration hasn't been started yet
(perform phase) the cancel request is just ignored. With this
patch, the request is remembered and as soon as perform phase
starts, migration is cancelled.

When the libvirt daemon is restarted it tries to reconnect to running
qemu domains. Since commit d38897a5 the
re-connection code runs in separate threads. In the original
implementation the maximum of domain ID's (that is used as an
initializer for numbering guests created next) while libvirt was
reconnecting to the guest.

With the threaded implementation this opens a possibility for race
conditions with the thread that is autostarting guests. When there's a
guest running with id 1 and the daemon is restarted. The autostart code
is reached first and spawns the first guest that should be autostarted
as id 1. This results into the following unwanted situation:

 # virsh list
   Id    Name                           State
  ----------------------------------------------------
   1     guest1                         running
   1     guest2                         running

This patch extracts the detection code before the re-connection threads
are started so that the maximum id of the guests being reconnected to is
known.

The only semantic change created by this is if the guest with greatest ID
quits before we are able to reconnect it's ID is used anyway as the
greatest one as without this patch the greatest ID of a process we could
successfuly reconnect to would be used.

Headers of qemuDomainSnapshotLoad and qemuDomainNetsRestart were
improperly formatted.

This patch adds support for external disk snapshots of inactive domains.
The snapshot is created by calling using qemu-img by calling:

 qemu-img create -f format_of_snapshot -o
 backing_file=/path/to/src,backing_fmt=format_of_backing_image
 /path/to/snapshot

in case the backing image format is known or probing is allowed and
otherwise:

 qemu-img create -f format_of_snapshot -o  backing_file=/path/to/src
 /path/to/snapshot

on each of the disks selected for snapshotting. This patch also modifies
the snapshot preparing function to support creating external snapshots
and to sanitize arguments. For now the user isn't able to mix external
and internal snapshots but this restriction might be lifted in the
future.

Some operations, APIs needs domain to be paused prior operation can be
performed, e.g. (managed-) save of a domain. The processors should be
restored in the end. However, if 'cont' fails for some reason, we log a
message but this is not sufficient as an event should be emitted as
well. Mgmt application can then decide what to do.

The code that was split out into the qemuDomainSaveMemory expands the
pointer containing the XML description of the domain that it gets from
higher layers. If the pointer changes the old one is invalid and the
upper layer function tries to free it causing an abort.

This patch changes the expansion of the original string to a new
allocation and copy of the contents.

Some of the pre-snapshot check have restrictions wired in regarding
configuration options that influence taking of external checkpoints.

This patch removes restrictions that would inhibit taking of such a
snapshot.

This patch adds support to take external system checkpoints.

The functionality is layered on top of the previous disk-only snapshot
code. When the checkpoint is requested the domain memory is saved to the
memory image file using migration to file. (The user may specify to
take the memory image while the guest is live with the
VIR_DOMAIN_SNAPSHOT_CREATE_LIVE flag.)

The memory save image shares format with the image created by
virDomainSave() API.

Before now, libvirt supported only internal snapshots for active guests.
This patch renames this function to qemuDomainSnapshotCreateActiveInternal
to prepare the grounds for external active snapshots.

The code that saves domain memory by migration to file can be reused
while doing external checkpoints of a machine. This patch extracts the
common code and places it in a separate function.

Two other places were left with the old code to look up snapshots.
Change them to use the snapshot lookup helper.

When pausing the guest while migration is running (to speed up
convergence) the virDomainSuspend API checks if the migration job is
active before entering the job. This could cause a possible race if the
virDomainSuspend is called while the job is active but ends before the
Suspend API enters the job (this would require that the migration is
aborted). This would cause a incorrect event to be emitted.

Both system checkpoint snapshots and disk snapshots were iterating
over all disks, doing a final sanity check before doing any work.
But since future patches will allow offline snapshots to be either
external or internal, it makes sense to share the pass over all
disks, and then relax restrictions in that pass as new modes are
implemented.  Future patches can then handle external disks when
the domain is offline, then handle offline --disk-snapshot, and
finally, combine with migration to file to gain a complete external
system checkpoint snapshot of an active domain without using 'savevm'.

* src/qemu/qemu_driver.c (qemuDomainSnapshotDiskPrepare)
(qemuDomainSnapshotIsAllowed): Merge...
(qemuDomainSnapshotPrepare): ...into one function.
(qemuDomainSnapshotCreateXML): Update caller.

Now that the XML supports listing internal snapshots, it is worth
always populating the <memory> and <disks> element to match.

* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateXML): Always
parse disk info and set memory info.

The libvirt coding standard is to use 'function(...args...)'
instead of 'function (...args...)'. A non-trivial number of
places did not follow this rule and are fixed in this patch.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

BZ:https://bugzilla.redhat.com/show_bug.cgi?id=871273
when using virsh qemu-attach to attach an existing qemu process,
if it misses the -M option in qemu command line, libvirtd crashed
because the NULL value of def->os.machine in later use.

Example:
/usr/libexec/qemu-kvm -name foo \
                      -cdrom /var/lib/libvirt/images/boot.img \
                      -monitor unix:/tmp/demo,server,nowait \

error: End of file while reading data: Input/output error
error: Failed to reconnect to the hypervisor

This patch tries to set default machine type if the value of
def->os.machine is still NULL after qemu command line parsing.

https://bugzilla.redhat.com/show_bug.cgi?id=871756

Commit cd1e8d1c assumed that systems new enough to have journald
also have mkostemp; but this is not true for uclibc.

For that matter, use of mkstemp[s] is unsafe in a multi-threaded
program.  We should prefer mkostemp[s] in the first place.

* bootstrap.conf (gnulib_modules): Add mkostemp, mkostemps; drop
mkstemp and mkstemps.
* cfg.mk (sc_prohibit_mkstemp): New syntax check.
* tools/virsh.c (vshEditWriteToTempFile): Adjust caller.
* src/qemu/qemu_driver.c (qemuDomainScreenshot)
(qemudDomainMemoryPeek): Likewise.
* src/secret/secret_driver.c (replaceFile): Likewise.
* src/vbox/vbox_tmpl.c (vboxDomainScreenshot): Likewise.

https://bugzilla.redhat.com/show_bug.cgi?id=871312

Recent fixes made almost all the right steps to make emulator pinned
to the cpuset of the whole domain in case <emulatorpin> isn't
specified, but qemudDomainGetEmulatorPinInfo still reports all the
CPUs even when cpuset is specified.  This patch fixes that.

Currently, we use iohelper when saving/restoring a domain.
However, if there's some kind of error (like I/O) it is not
propagated to libvirt. Since it is not qemu who is doing
the actual write() it will not get error. The iohelper does.
Therefore we should check for iohelper errors as it makes
libvirt more user friendly.

https://bugzilla.redhat.com/show_bug.cgi?id=636832

When using block copy to pivot over to a new chain, the backing files
for the new chain might still need labeling (particularly if the user
passes --reuse-ext with a relative backing file name).  Relabeling a
file that is already labeled won't hurt, so this just labels the entire
chain at the point of the pivot.  Doing the relabel of the chain uses
the fact that we already safely probed the file type of an external
file at the start of the block copy.

* src/qemu/qemu_driver.c (qemuDomainBlockPivot): Relabel chain before
asking qemu to pivot.

Use the recent addition of qemuDomainPrepareDiskChainElement to
obtain locking manager lease, permit a block device through cgroups,
and set the SELinux label; then audit the fact that we hand a new
file over to qemu.  Alas, releasing the lease and label at the end
of the mirroring is a trickier prospect (we would have to trace the
backing chain of both source and destination, and be sure not to
revoke rights to any part of the chain that is shared), so for now,
virDomainBlockJobAbort still leaves things with additional access
granted (as block-pull and block-commit have the same problem of
not clamping access after completion, a future cleanup would cover
all three commands).

* src/qemu/qemu_driver.c (qemuDomainBlockCopy): Set up labeling.

Support the REUSE_EXT flag, in part by copying sanity checks from
snapshot code.  This code introduces a case of probing an external
file for its type; such an action would be a security risk if the
existing file is supposed to be raw but the contents resemble some
other format; however, since the virDomainBlockRebase API has a
flag to force treating the file as raw rather than probe, we can
assume that probing is safe in all other instances.  Besides, if
we don't probe or force raw, then qemu will.

* src/qemu/qemu_driver.c (qemuDomainBlockRebase): Allow REUSE_EXT
flag.
(qemuDomainBlockCopy): Wire up flag, and add some sanity checks.

Minimal patch to wire up all the pieces in the previous patches
to actually enable a block copy job.  By minimal, I mean that
qemu creates the file (that is, no REUSE_EXT flag support yet),
SELinux must be disabled, a lock manager is not informed, and the
audit logs aren't updated.  But those will be added as
improvements in future patches.

This patch is designed so that if we ever add a future API
virDomainBlockCopy with more bells and whistles (such as letting
the user specify a destination image format different than the
source), where virDomainBlockRebase is a wrapper around the
simpler portions of the new functionality, then the new API can
just reuse the new qemuDomainBlockCopy function and already
support _SHALLOW and _REUSE_EXT flags.  Also note that libvirt.c
already filtered the new flags if _COPY is not present, so that
we are not impacting the case of BlockRebase being a wrapper
around BlockPull.

* src/qemu/qemu_driver.c (qemuDomainBlockCopy): New function.
(qemuDomainBlockRebase): Call it when appropriate.

Since libvirt drops locks between issuing a monitor command and
getting a response, it is possible for libvirtd to be restarted
before getting a response on a block-job-complete command; worse, it
is also possible for the guest to shut itself down during the window
while libvirtd is down, ending the qemu process.  A management app
needs to know if the pivot happened (and the destination file
contains guest contents not in the source) or failed (and the source
file contains guest contents not in the destination), but since
the job is finished, 'query-block-jobs' no longer tracks the
status of the job, and if the qemu process itself has disappeared,
even 'query-block' cannot be checked to ask qemu its current state.

At the time of this patch, the design for persistent bitmap has not
been clarified, so a followup patch will be needed once qemu
actually figures out how to expose it, and we figure out how to use
it.  In the meantime, we have a solution that avoids the worst of
the problem.  [This problem was first analyzed with the RHEL 6.3
__com.redhat_drive-reopen command; which partly explains why
upstream qemu 1.3 ditched the drive-reopen idea and went with
block-job-complete plus persistent bitmap instead.]

If we surround 'drive-reopen' with a pause/resume pair, then we can
guarantee that the guest cannot modify either source or destination
files in the window of libvirtd uncertainty, and the management app
is guaranteed that either libvirt knows the outcome and reported it
correctly; or that on libvirtd restart, the guest will still be
paused and that the qemu process cannot have disappeared due to
guest shutdown; and use that as a clue that the management app must
implement recovery protocol, with both source and destination files
still being in sync and with 'query-block' still being an option as
part of that recovery.  My testing shows that the pause window will
typically be only a fraction of a second.

* src/qemu/qemu_driver.c (qemuDomainBlockPivot): Pause around
drive-reopen.
(qemuDomainBlockJobImpl): Update caller.

This is the bare minimum to end a copy job (of course, until a
later patch adds the ability to start a copy job, this patch
doesn't do much in isolation; I've just split the patches to
ease the review).

This patch intentionally avoids SELinux, lock manager, and audit
actions.  Also, if libvirtd restarts at the exact moment that a
'block-job-complete' is in flight, the proposed proper way to
detect the outcome of that would be with a persistent bitmap and
some additional query commands when libvirtd restarts.  This
patch is enough to test the common case of success when used
correctly, while saving the subtleties of proper cleanup for
worst-case errors for later.

When a mirror job is started, cancelling the job safely reverts back
to the source disk, regardless of whether the destination is in
phase 1 (streaming, in which case the destination is worthless) or
phase 2 (mirroring, in which case the destination is synced up to
the source at the time of the cancel).  Our existing code does just
fine in either phase, other than some bookkeeping cleanup; this
implements live block copy.

Ideas for future enhancements via new flags:

Depending on when persistent bitmap support is added, it may be
worth adding a VIR_DOMAIN_REBASE_COPY_ATOMIC flag that fails up
front if we detect an older qemu with risky pivot operation.

Interesting side note: while snapshot-create --disk-only creates a
copy of the disk at a point in time by moving the domain on to a
new file (the copy is the file now in the just-extended backing
chain), blockjob --abort of a copy job creates a copy of the disk
while keeping the domain on the original file.  There may be
potential improvements to the snapshot code to exploit block copy
over multiple disks all at one point in time.  And, if
'block-job-cancel' were made part of 'transaction', you could
copy multiple disks at the same point in time without pausing
the domain.  This also implies we may want to add a --quiesce flag
to virDomainBlockJobAbort, so that when breaking a mirror (whether
by cancel or pivot), the side of the mirror that we are abandoning
is at least in a stable state with regards to guest I/O.

* src/qemu/qemu_driver.c (qemuDomainBlockJobAbort): Accept new flag.
(qemuDomainBlockPivot): New helper function.
(qemuDomainBlockJobImpl): Implement it.

Handle the new type of block copy event and info.  Of course,
this patch does nothing until a later patch actually allows the
creation/abort of a block copy job.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_BLOCK_JOB_READY): New
block job status.
* src/libvirt.c (virDomainBlockRebase): Document the event.
* src/qemu/qemu_monitor_json.c (eventHandlers): New event.
(qemuMonitorJSONHandleBlockJobReady): New function.
(qemuMonitorJSONGetBlockJobInfoOne): Translate new job type.
(qemuMonitorJSONHandleBlockJobImpl): Handle new event and job type.
* src/qemu/qemu_process.c (qemuProcessHandleBlockJob): Recognize
the event to minimize snooping.
* src/qemu/qemu_driver.c (qemuDomainBlockJobImpl): Snoop a successful
info query to save effort on a pivot request.

For now, disk migration via block copy job is not implemented in
libvirt.  But when we do implement it, we have to deal with the
fact that qemu does not yet provide an easy way to re-start a qemu
process with mirroring still intact.  Paolo has proposed an idea
for a persistent dirty bitmap that might make this possible, but
until that design is complete, it's hard to say what changes
libvirt would need.  Even something like 'virDomainSave' becomes
hairy, if you realize the implications that 'virDomainRestore'
would be stuck with recreating the same mirror layout.

But if we step back and look at the bigger picture, we realize that
the initial client of live storage migration via disk mirroring is
oVirt, which always uses transient domains, and that if a transient
domain is destroyed while a mirror exists, oVirt can easily restart
the storage migration by creating a new domain that visits just the
source storage, with no loss in data.

We can make life a lot easier by being cowards for now, forbidding
certain operations on a domain.  This patch guarantees that we
never get in a state where we would have to restart a domain with
a mirroring block copy, by preventing saves, snapshots, migration,
hot unplug of a disk in use, and conversion to a persistent domain
(thankfully, it is still relatively easy to 'virsh undefine' a
running domain to temporarily make it transient, run tests on
'virsh blockcopy', then 'virsh define' to restore the persistence).
Later, if the qemu design is enhanced, we can relax our code.

The change to qemudDomainDefine looks a bit odd for undoing an
assignment, rather than probing up front to avoid the assignment,
but this is because of how virDomainAssignDef combines both a
lookup and assignment into a single function call.

* src/conf/domain_conf.h (virDomainHasDiskMirror): New prototype.
* src/conf/domain_conf.c (virDomainHasDiskMirror): New function.
* src/libvirt_private.syms (domain_conf.h): Export it.
* src/qemu/qemu_driver.c (qemuDomainSaveInternal)
(qemuDomainSnapshotCreateXML, qemuDomainRevertToSnapshot)
(qemuDomainBlockJobImpl, qemudDomainDefine): Prevent dangerous
actions while block copy is already in action.
* src/qemu/qemu_hotplug.c (qemuDomainDetachDiskDevice): Likewise.
* src/qemu/qemu_migration.c (qemuMigrationIsAllowed): Likewise.

This resolves:

   https://bugzilla.redhat.com/show_bug.cgi?id=862515

which describes inconsistencies in dealing with duplicate mac
addresses on network devices in a domain.

(at any rate, it resolves *almost* everything, and prints out an
informative error message for the one problem that isn't solved, but
has a workaround.)

A synopsis of the problems:

1) you can't do a persistent attach-interface of a device with a mac
address that matches an existing device.

2) you *can* do a live attach-interface of such a device.

3) you *can* directly edit a domain and put in two devices with
matching mac addresses.

4) When running virsh detach-device (live or config), only MAC address
is checked when matching the device to remove, so the first device
with the desired mac address will be removed. This isn't always the
one that's wanted.

5) when running virsh detach-interface (live or config), the only two
items that can be specified to match against are mac address and model
type (virtio, etc) - if multiple netdevs match both of those
attributes, it again just finds the first one added and assumes that
is the only match.

Since it is completely valid to have multiple network devices with the
same MAC address (although it can cause problems in many cases, there
*are* valid use cases), what is needed is:

1) remove the restriction that prohibits doing a persistent add of a
netdev with a duplicate mac address.

2) enhance the backend of virDomainDetachDeviceFlags to check for
something that *is* guaranteed unique (but still work with just mac
address, as long as it yields only a single results.

This patch does three things:

1) removes the check for duplicate mac address during a persistent
netdev attach.

2) unifies the searching for both live and config detach of netdevices
in the subordinate functions of qemuDomainModifyDeviceFlags() to use the
new function virDomainNetFindIdx (which matches mac address and PCI
address if available, checking for duplicates if only mac address was
specified). This function returns -2 if multiple matches are found,
allowing the callers to print out an appropriate message.

Steps 1 & 2 are enough to fully fix the problem when using virsh
attach-device and detach-device (which require an XML description of
the device rather than a bunch of commandline args)

3) modifies the virsh detach-interface command to check for multiple
matches of mac address and show an error message suggesting use of the
detach-device command in cases where there are multiple matching mac
addresses.

Later we should decide how we want to input a PCI address on the virsh
commandline, and enhance detach-interface to take a --address option,
eliminating the need to use detach-device

* src/conf/domain_conf.c
* src/conf/domain_conf.h
* src/libvirt_private.syms
  * added new virDomainNetFindIdx function
  * removed now unused virDomainNetIndexByMac and
    virDomainNetRemoveByMac

* src/qemu/qemu_driver.c
  * remove check for duplicate max from qemuDomainAttachDeviceConfig
  * use virDomainNetFindIdx/virDomainNetRemove instead
    of virDomainNetRemoveByMac in qemuDomainDetachDeviceConfig
  * use virDomainNetFindIdx instead of virDomainIndexByMac
    in qemuDomainUpdateDeviceConfig

* src/qemu/qemu_hotplug.c
  * use virDomainNetFindIdx instead of a homespun loop in
    qemuDomainDetachNetDevice.

* tools/virsh-domain.c: modified detach-interface command as described
    above

It turns out that the cpuacct results properly account for offline
cpus, and always returns results for every possible cpu, not just
the online ones.  So there is no need to check the map of online
cpus in the first place, merely only a need to know the maximum
possible cpu.  Meanwhile, virNodeGetCPUBitmap had a subtle change
from returning the maximum id to instead returning the width of
the bitmap (one larger than the maximum id) in commit 2f4c5338,
which made this code encounter some off-by-one logic leading to
bad error messages when a cpu was offline:

$ virsh cpu-stats dom
error: Failed to virDomainGetCPUStats()

error: An error occurred, but the cause is unknown

Cleaning this up unraveled a chain of other unused variables.

* src/qemu/qemu_driver.c (qemuDomainGetPercpuStats): Drop
pointless check for cpumap changes, and use correct number of
cpus.  Simplify signature.
(qemuDomainGetCPUStats): Adjust caller.
* src/nodeinfo.h (nodeGetCPUCount): New prototype.
(nodeGetCPUBitmap): Drop unused parameter.
* src/nodeinfo.c (nodeGetCPUBitmap): Likewise.
(nodeGetCPUMap): Adjust caller.
(nodeGetCPUCount): New function.
* src/libvirt_private.syms (nodeinfo.h): Export it.

Driver support added for:
- test: pretending 8 host CPUS, 3 being online
- qemu, lxc, openvz, uml: using nodeGetCPUMap
Signed-off-by: NViktor Mihajlovski <mihajlov@linux.vnet.ibm.com>

Callers should not need to know what the name of the file to
be read in the Linux-specific version of nodeGetCPUmap;
furthermore, qemu cares about online cpus, not present cpus,
when determining which cpus to skip.

While at it, I fixed the fact that we were computing the maximum
online cpu id by doing a slow iteration, when what we really want
to know is the max available cpu.

* src/nodeinfo.h (nodeGetCPUmap): Rename...
(nodeGetCPUBitmap): ...and simplify signature.
* src/nodeinfo.c (linuxParseCPUmax): New function.
(linuxParseCPUmap): Simplify and alter signature.
(nodeGetCPUBitmap): Change implementation.
* src/libvirt_private.syms (nodeinfo.h): Reflect rename.
* src/qemu/qemu_driver.c (qemuDomainGetPercpuStats): Update
caller.

On one hand, numad probably will manage the affinity of domain process
dynamically in future. On the other hand, even numad won't manage it,
it still could confusion. Let's make things simpler enough to avoid
the lair for now.

The snapshot code when reusing an existing file had hard-to-read
logic, as well as a missing sanity check: REUSE_EXT should require
the destination to already be present.

* src/qemu/qemu_driver.c (qemuDomainSnapshotDiskPrepare): Require
destination on REUSE_EXT, rename variable for legibility.

Yet another instance of where using plain open() mishandles files
that live on root-squash NFS, and where improving the API can
improve the chance of a successful probe.

* src/util/storage_file.h (virStorageFileProbeFormat): Alter
signature.
* src/util/storage_file.c (virStorageFileProbeFormat): Use better
method for opening file.
* src/qemu/qemu_driver.c (qemuDomainGetBlockInfo): Update caller.
* src/storage/storage_backend_fs.c (virStorageBackendProbeTarget):
Likewise.

In v2 migration protocol, XML is obtained by calling domainGetXMLDesc.
This includes the default USB controller in XML, which breaks migration
to older libvirt (before 0.9.2).

Commit 409b5f54
    qemu: Emit compatible XML when migrating a domain
only fixed this for v3 migration.

This patch uses the new VIR_DOMAIN_XML_MIGRATABLE flag (detected by
VIR_DRV_FEATURE_XML_MIGRATABLE) to obtain XML without the default controller,
enabling backward v2 migration.

Gcc with optimization warns:
../../src/qemu/qemu_driver.c: In function 'qemuDomainBlockCommit':
../../src/qemu/qemu_driver.c:12813:46: error: 'disk' may be used uninitialized in this function [-Werror=maybe-uninitialized]
../../src/qemu/qemu_driver.c:12698:25: note: 'disk' was declared here
cc1: all warnings being treated as errors

so obviously I had only been testing with optimization off.

* src/qemu/qemu_driver.c (qemuDomainBlockCommit): Guard cleanup.

I finally have all the pieces in place to perform a block-commit with
SELinux enforcing.  There's still missing cleanup work when the commit
completes, but doing that requires tracking both the backing chain and
the base and top files within that chain in domain XML across libvirtd
restarts.  Furthermore, from a security standpoint, once you have
granted access, you must assume any damage that can be done will be
done; later revoking access is nice to minimize the window of damage,
but less important as it does not affect the fact that damage can be
done in the first place.  Therefore, deferring the revoke efforts until
we have better XML tracking of what chain operations are in effect,
including across a libvirtd restart, is reasonable.

* src/qemu/qemu_driver.c (qemuDomainBlockCommit): Label disks as
needed.
(qemuDomainPrepareDiskChainElement): Cast away const.

Previously, snapshot code did its own permission granting (lock
manager, cgroup device controller, and security manager labeling)
inline.  But now that we are adding block-commit and block-copy
which also have to change permissions, it's better to reuse
common code for the task.  While snapshot should fall back to
no access if read-write access failed, block-commit will want to
fall back to read-only access.  The common code doesn't know
whether failure to grant read-write access should revert to no
access (snapshot, block-copy) or read-only access (block-commit).
This code can also be used to revoke access to unused files after
block-pull.

It might be nice to clean things up in a future patch by adding
new functions to the lock manager, cgroup manager, and security
manager that takes a single file name and applies context of a
disk to that file, rather than the current semantics of applying
context to the entire chain already associated to a disk.  That
way, we could avoid the games this patch plays of temporarily
swapping out the disk->src and related fields of the disk.  But
that would involve more code changes, so this patch really is
the smallest hack for doing the necessary work; besides, this
patch is more or less code motion (the hack was already employed
by the snapshot creation code, we are just making it reusable).

* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateSingleDiskActive)
(qemuDomainSnapshotUndoSingleDiskActive): Refactor labeling hacks...
(qemuDomainPrepareDiskChainElement): ...into new function.