In order to distinguish disk snapshots from system checkpoints, a
new state value that is only valid for snapshots is helpful.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_LAST): New placeholder.
* src/conf/domain_conf.h (virDomainSnapshotState): New enum mapping.
(VIR_DOMAIN_DISK_SNAPSHOT): New internal enum value.
* src/conf/domain_conf.c (virDomainState): Use placeholder.
(virDomainSnapshotState): Extend mapping by one for use in snapshot.
(virDomainSnapshotDefParseString, virDomainSnapshotDefFormat):
Handle new state.
(virDomainObjSetState, virDomainStateReasonToString)
(virDomainStateReasonFromString): Avoid compiler warnings.
* tools/virsh.c (vshDomainState, vshDomainStateReasonToString):
Likewise.
* src/libvirt_private.syms (domain_conf.h): Export new functions.
* docs/schemas/domainsnapshot.rng: Tighten state definition.
* docs/formatsnapshot.html.in: Document it.
* tests/domainsnapshotxml2xmlout/disk_snapshot.xml: New test.

Easy enough to emulate even with older servers.

* tools/virsh.c (cmdSnapshotCreate, cmdSnapshotCreateAs): Add
--halt flag.
(vshSnapshotCreate): Emulate halt when flag is unsupported.
* tools/virsh.pod (snapshot-create, snapshot-create-as): Document
it.

Since a snapshot is fully recoverable, it is useful to have a
snapshot as a means of hibernating a guest, then reverting to
the snapshot to wake the guest up.  This mode of usage is
similar to 'virsh save/virsh restore', except that virsh
save uses an external file while virsh snapshot keeps the
vm state internal to a qcow2 file.  However, it only works on
persistent domains.

In the usage pattern of snapshot/revert for hibernating a guest,
there is no need to keep the guest running between the two points
in time, especially since that would generate runtime state that
would just be discarded.  Add a flag to make it possible to
stop the domain after the snapshot has completed.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_SNAPSHOT_CREATE_HALT):
New flag.
* src/libvirt.c (virDomainSnapshotCreateXML): Document it.
* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateXML)
(qemuDomainSnapshotCreateActive): Implement it.

It would technically be possible to have virsh compute the list
of descendants of a given snapshot, then delete those one at
a time.  But it's complex, and not worth writing for a first
cut at implementing the new flags.

* tools/virsh.c (cmdSnapshotDelete): Add --children-only,
--metadata.
* tools/virsh.pod (snapshot-delete): Document them.

Reverting to a state prior to an external snapshot risks
corrupting any other branches in the snapshot hierarchy that
were using the snapshot as a read-only backing file.  So
disk snapshot code will default to preventing reverting to
a snapshot that has any children, meaning that deleting just
the children of a snapshot becomes a useful operation in
preparing that snapshot for being a future reversion target.
The code for the new flag is simple - it's one less deletion,
plus a tweak to keep the current snapshot correct.

* include/libvirt/libvirt.h.in
(VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN_ONLY): New flag.
* src/libvirt.c (virDomainSnapshotDelete): Document it, and
enforce mutual exclusion.
* src/qemu/qemu_driver.c (qemuDomainSnapshotDelete): Implement
it.

The previous patch introduced new config, but if a hypervisor does
not support that new config, someone can write XML that does not
behave as documented.  This prevents some of those cases by
explicitly rejecting transient disks for several hypervisors.

Disk snapshots will require a new flag to actually affect a snapshot
creation, so there's not much to reject there.

* src/qemu/qemu_command.c (qemuBuildDriveStr): Reject transient
disks for now.
* src/libxl/libxl_conf.c (libxlMakeDisk): Likewise.
* src/xenxs/xen_sxpr.c (xenFormatSxprDisk): Likewise.
* src/xenxs/xen_xm.c (xenFormatXMDisk): Likewise.

As discussed here:
https://www.redhat.com/archives/libvir-list/2011-August/msg00361.html
https://www.redhat.com/archives/libvir-list/2011-August/msg00552.html

Adds snapshot attribute and transient sub-element:

<devices>
  <disk type=... snapshot='no|internal|external'>
    ...
    <transient/>
  </disk>
</devices>

* docs/schemas/domaincommon.rng (snapshot): New define.
(disk): Add snapshot and persistent attributes.
* docs/formatdomain.html.in: Document them.
* src/conf/domain_conf.h (virDomainDiskSnapshot): New enum.
(_virDomainDiskDef): New fields.
* tests/qemuxml2argvdata/qemuxml2argv-disk-transient.xml: New
test of rng, no args counterpart until qemu support is complete.
* tests/qemuxml2argvdata/qemuxml2argv-disk-snapshot.args: New
file, snapshot attribute does not affect args.
* tests/qemuxml2argvdata/qemuxml2argv-disk-snapshot.xml: Likewise.
* tests/qemuxml2argvtest.c (mymain): Run new test.

Fix bug #611823 storage driver should prohibit pools with duplicate
underlying storage.

Add internal API virStoragePoolSourceFindDuplicate() to do uniqueness
check based on source location infomation for pool type.

* AUTHORS: add Lei Li

Fix qemuAssignDevicePCISlots() and the associated regression tests

Current code reserves slot 1 function 2 even if there is a user
defined PIIX3 USB controller there.

At least Xen-3.4.3 translates the /vm/localtime SXPR value to
/domain/platform/localtime and /domain/image/{linux,hvm}/localtime when
the domain is defined.  When reading back that information libvirt only
handles HVM domains, but not PV domains: This results in libvirtd always
returning
    <clock offset="utc"/>
while Xend used (localtime 1).

For PV domains use /domain/image/linux/localtime.

When reverting to a snapshot, the inactive domain configuration
has to be rolled back to what it was at the time of the snapshot.
Additionally, if the VM is active and the snapshot was active,
this now adds a failure if the two configurations are ABI
incompatible, rather than risking qemu confusion.

A future patch will add a VIR_DOMAIN_SNAPSHOT_FORCE flag, which
will be required for two risky code paths - reverting to an
older snapshot that lacked full domain information, and reverting
from running to a live snapshot that requires starting a new qemu
process.  Any reverting that stops a running vm is also a form
of data loss (discarding the current running state to go back in
time), but as that is what reversion usually implies, it is
probably not worth requiring a force flag.

* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateXML): Copy out
domain.
(qemuDomainSnapshotCreateXML, qemuDomainRevertToSnapshot): Perform
ABI compatibility checks.

This patch will probably cause merge conflicts to those trying
to do backports.  The end goal is simple - domaincommon.rng
should be the state of domain.rng pre-patch, with a few lines
tweaked in the header, while domain.rng post-patch is now just
a shell that includes domaincommon.rng and sets the <start>.

* docs/schemas/domain.rng: Move guts...
* docs/schemas/domaincommon.rng: ...to new file.
* docs/schemas/domainsnapshot.rng: Allow new xml.
* docs/schemas/Makefile.am (schema_DATA): Distribute new file.
* tests/domainsnapshotxml2xmlout/full_domain.xml: New test.
* libvirt.spec.in (%files client): Ship new file.  Sort lines.
* mingw32-libvirt.spec.in: Likewise.

Commit 69278878 fixed one direction of arbitrarily-named snapshots,
but not the round trip path.  While auditing domain_conf, I found
a couple other instances that weren't escaping arbitrary strings.

* src/conf/domain_conf.c (virDomainFSDefFormat)
(virDomainGraphicsListenDefFormat, virDomainSnapshotDefFormat):
Escape arbitrary strings.

Just like VM saved state images (virsh save), snapshots MUST
track the inactive domain xml to detect any ABI incompatibilities.

The indentation is not perfect, but functionality comes before form.

Later patches will actually supply a full domain; for now, this
wires up the storage to support one, but doesn't ever generate one
in dumpxml output.

Happily, libvirt.c was already rejecting use of VIR_DOMAIN_XML_SECURE
from read-only connections, even though before this patch, there was
no information to be secured by the use of that flag.

And while we're at it, mark the libvirt snapshot metadata files
as internal-use only.

* src/libvirt.c (virDomainSnapshotGetXMLDesc): Document flag.
* src/conf/domain_conf.h (_virDomainSnapshotDef): Add member.
(virDomainSnapshotDefParseString, virDomainSnapshotDefFormat):
Update signature.
* src/conf/domain_conf.c (virDomainSnapshotDefFree): Clean up.
(virDomainSnapshotDefParseString): Optionally parse domain.
(virDomainSnapshotDefFormat): Output full domain.
* src/esx/esx_driver.c (esxDomainSnapshotCreateXML)
(esxDomainSnapshotGetXMLDesc): Update callers.
* src/vbox/vbox_tmpl.c (vboxDomainSnapshotCreateXML)
(vboxDomainSnapshotGetXMLDesc): Likewise.
* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateXML)
(qemuDomainSnapshotLoad, qemuDomainSnapshotGetXMLDesc)
(qemuDomainSnapshotWriteMetadata): Likewise.
* docs/formatsnapshot.html.in: Rework doc example.
Based on a patch by Philipp Hahn.

Minor semantic change - allow domain xml to be generated in place
within a larger buffer, rather than having to go through a
temporary string.

* src/conf/domain_conf.c (virDomainDefFormatInternal): Add
parameter.
(virDomainDefFormat, virDomainObjFormat): Update callers.

Migration is another case of stranding metadata.  And since
snapshot metadata is arbitrarily large, there's no way to
shoehorn it into the migration cookie of migration v3.

This patch consolidates two existing locations for migration
validation into one helper function, then enhances that function
to also do the new checks.  If we could always trust the source
to validate migration, then the destination would not have to
do anything; but since older servers that did not do checking
can migrate to newer destinations, we have to repeat some of
the same checks on the destination; meanwhile, we want to
detect failures as soon as possible.  With migration v2, this
means that validation will reject things at Prepare on the
destination if the XML exposes the problem, otherwise at Perform
on the source; with migration v3, this means that validation
will reject things at Begin on the source, or if the source
is old and the XML exposes the problem, then at Prepare on the
destination.

This patch is necessarily over-strict.  Once a later patch
properly handles auto-cleanup of snapshot metadata on the
death of a transient domain, then the only time we actually
need snapshots to prevent migration is when using the
--undefinesource flag on a persistent source domain.

It is possible to recreate snapshot metadata on the destination
with VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE and
VIR_DOMAIN_SNAPSHOT_CREATE_CURRENT.  But for now, that is limited,
since if we delete the snapshot metadata prior to migration,
then we won't know the name of the current snapshot to pass
along; and if we delete the snapshot metadata after migration
and use the v3 migration cookie to pass along the name of the
current snapshot, then we need a way to bypass the fact that
this patch refuses migration with snapshot metadata present.

So eventually, we may have to introduce migration protocol v4
that allows feature negotiation and an arbitrary number of
handshake exchanges, so as to pass as many rpc calls as needed
to transfer all the snapshot xml hierarchy.

But all of that is thoughts for the future; for now, the best
course of action is to quit early, rather than get into a
funky state of stale metadata; then relax restrictions later.

* src/qemu/qemu_migration.h (qemuMigrationIsAllowed): Make static.
* src/qemu/qemu_migration.c (qemuMigrationIsAllowed): Alter
signature, and allow checks for both outgoing and incoming.
(qemuMigrationBegin, qemuMigrationPrepareAny)
(qemuMigrationPerformJob): Update callers.

A nice benefit of deleting all snapshots at undefine time is that
you don't have to do any reparenting or subtree identification - since
everything goes, this is an O(n) process, whereas using multiple
virDomainSnapshotDelete calls would be O(n^2) or worse.  But it is
only doable for snapshot metadata, where we are in control of the
data being deleted; for the actual snapshots, there's too much
likelihood of something going wrong, and requiring even more API
calls to figure out what failed in the meantime, so callers are
better off deleting the snapshot data themselves one snapshot at
a time where they can deal with failures as they happen.

* src/qemu/qemu_driver.c (qemuDomainUndefineFlags): Honor new flags.

As more clients start to want to know this information, doing
a PATH stat walk and malloc for every client adds up.

We are only caching the location, not the capabilities, so even
if qemu-img is updated in the meantime, it will still probably
live in the same location.  So there is no need to worry about
clearing this particular cache.

* src/qemu/qemu_conf.h (qemud_driver): Add member.
* src/qemu/qemu_driver.c (qemudShutdown): Cleanup.
(qemuFindQemuImgBinary): Add an argument, and cache result.
(qemuDomainSnapshotForEachQcow2, qemuDomainSnapshotDiscard)
(qemuDomainSnapshotCreateInactive, qemuDomainSnapshotRevertInactive)
(qemuDomainSnapshotCreateXML, qemuDomainRevertToSnapshot): Update
callers.

Prepare for code sharing.  No semantic change.

* src/qemu/qemu_driver.c (qemuFindQemuImgBinary)
(qemuDomainSnapshotWriteMetadata)
(qemuDomainSnapshotDiscard)
(qemuDomainSnapshotForEachQcow2): Float up.
(qemuDomainSnapshotDiscardDescendant): Likewise, and rename...
(qemuDomainSnapshotDiscardAll): ...for generic use.
(qemuDomainSnapshotDelete): Update caller.

Similar to 'undefine --managed-save' (commit 83e849c1), we must
assume that the old API is unsafe; however, we cannot emulate
metadata-only deletion on older servers.  Additionally, we have
the wrinkle that while virDomainUndefineFlags and managed save
cleanup were introduced in 0.9.4, it wasn't until 0.9.5 that
snapshots block undefine of a domain.  Do the best we can given
the server we are talking to.

* tools/virsh.c (cmdUndefine): Add --snapshots-metadata flag.
* tools/virsh.pod (undefine, destroy, shutdown): Document effect
of snapshots.

Just as leaving managed save metadata behind can cause problems
when creating a new domain that happens to collide with the name
of the just-deleted domain, the same is true of leaving any
snapshot metadata behind.  For safety sake, extend the semantic
change of commit b26a9fa9 to also cover snapshot metadata as a
reason to reject undefining an inactive domain.  A future patch
will make sure that shutdown of a transient domain automatically
deletes snapshot metadata (whether by destroy, shutdown, or
guest-initiated action).  Management apps of transient domains
should take care to capture xml of snapshots, if it is necessary
to recreate the snapshot metadata on a later transient domain
with the same name and uuid.

This also documents a new flag that hypervisors can choose to
support as a shortcut for taking care of the metadata as part of
the undefine process; however, nontrivial driver support for these
flags will be deferred to future patches.

Note that ESX and VBox can never be transient; therefore, they
do not have to worry about automatic cleanup after shutdown
(the persistent domain still remains); likewise they never
store snapshot metadata, so the undefine flag is trivial.
The nontrivial work remaining is thus in the qemu driver.

* include/libvirt/libvirt.h.in
(VIR_DOMAIN_UNDEFINE_SNAPSHOTS_METADATA): New flag.
* src/libvirt.c (virDomainUndefine, virDomainUndefineFlags):
Document new limitations and flag.
* src/esx/esx_driver.c (esxDomainUndefineFlags): Trivial
implementation.
* src/vbox/vbox_tmpl.c (vboxDomainUndefineFlags): Likewise.
* src/qemu/qemu_driver.c (qemuDomainUndefineFlags): Enforce
the limitations.

Redefining a qemu snapshot requires a bit of a tweak to the common
snapshot parsing code, but the end result is quite nice.

Be careful that redefinitions do not introduce circular parent
chains.  Also, we don't want to allow conversion between online
and offline existing snapshots.  We could probably do some more
validation for snapshots that don't already exist to make sure
they are even feasible, by parsing qemu-img output, but that
can come later.

* src/conf/domain_conf.h (virDomainSnapshotParseFlags): New
internal flags.
* src/conf/domain_conf.c (virDomainSnapshotDefParseString): Alter
signature to take internal flags.
* src/esx/esx_driver.c (esxDomainSnapshotCreateXML): Update caller.
* src/vbox/vbox_tmpl.c (vboxDomainSnapshotCreateXML): Likewise.
* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateXML): Support
new public flags.

Supporting NO_METADATA on snapshot creation is interesting - we must
still return a valid opaque snapshot object, but the user can't get
anything out of it (unless we add a virDomainSnapshotGetName()),
since it is no longer registered with the domain.

Also, virsh now tries to query for secure xml, in anticipation of
when we store <domain> xml inside <domainsnapshot>; for now, we
can trivially support it, since we have nothing secure.

* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateXML): Support
new flag.
(qemuDomainSnapshotGetXMLDesc): Trivially support VIR_DOMAIN_XML_SECURE.

Wire up the new snapshot creation flags in virsh.  For convenience,
teach 'snapshot-current' how to make an existing snapshot become
current (can be used after upgrading to newer libvirt to recover
from the fact that the older libvirt lost track of the current
snapshot after a restart).  The snapshot-create-as command is
intentionally not taught --redefine or --current, as this would
imply adding a lot of other options for everything else that can
appear in the <domainsnapshot> xml, but which is normally read-only.
Besides, redefining will usually be done on files created by
snapshot-dumpxml, rather than something built up by hand on the
command line.  And now that we can redefine, we can edit.

* tools/virsh.c (cmdSnapshotCreate): Add --redefine, --current,
and --no-metadata.
(cmdSnapshotCreateAs): Add --no-metadata.
(cmdSnapshotCurrent): Add snapshotname to alter current snapshot.
(cmdSnapshotEdit): New command.
* tools/virsh.pod (snapshot-create, snapshot-create-as)
(snapshot-current, snapshot-edit):  Document these.

The next patch will make snapshot creation more complex, so it's
better to avoid repetition of the complexity.

* tools/virsh.c (vshSnapshotCreate): New helper function.
(cmdSnapshotCreate, cmdSnapshotCreateAs): Use it.

The first two flags are essential for being able to replicate
snapshot hierarchies across multiple hosts, which will come in
handy for supervised migrations.  It also allows a management app
to take a snapshot of a transient domain, save the metadata, stop
the domain, recreate a new transient domain by the same name,
redefine the snapshot, then revert to it.

This is not quite as convenient as leaving the metadata behind
after a domain is no longer around, but doing that has a few
problems: 1. the libvirt API can only delete snapshot metadata
if there is a valid domain handle to use to get to that snapshot
object - if stale data is left behind without a domain, there is
no way to request that the data be cleaned up. 2. creating a new
domain with the same name but different uuid than the older
domain where a snapshot existed cannot use the older snapshot
data; this risks confusing libvirt, and forbidding the stale
data is similar to the recent patch to forbid stale managed save.

The first two flags might be useful on hypervisors with no metadata,
but only for modifying the notion of the current snapshot;
however, I don't know how to do that for ESX or VBox.

The third flag is a convenience option, to combine a creation with
a delete metadata into one step.  It is trivial for hypervisors
with no metadata.

The qemu changes will be involved enough to warrant a separate patch.

* include/libvirt/libvirt.h.in
(VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE)
(VIR_DOMAIN_SNAPSHOT_CREATE_CURRENT)
(VIR_DOMAIN_SNAPSHOT_CREATE_NO_METADATA): New flags.
* src/libvirt.c (virDomainSnapshotCreateXML): Document them, and
enforce mutual exclusion.
* src/esx/esx_driver.c (esxDomainSnapshotCreateXML): Trivial
implementation.
* src/vbox/vbox_tmpl.c (vboxDomainSnapshotCreateXML): Likewise.
* docs/formatsnapshot.html.in: Document re-creation.

Filtering for roots is pretty easy to do.

* src/conf/domain_conf.h (virDomainSnapshotObjListGetNames)
(virDomainSnapshotObjListNum): Update prototype.
* src/conf/domain_conf.c (virDomainSnapshotObjListCopyNames)
(virDomainSnapshotObjListGetNames, virDomainSnapshotObjListCount)
(virDomainSnapshotObjListNum): Support filtering.
* src/qemu/qemu_driver.c (qemuDomainSnapshotNum)
(qemuDomainSnapshotListNames): Update callers.

New flag bits are worth exposing via virsh.  In the case of
snapshot-list --roots, it's possible to emulate this even when
talking to an older server that lacks the bit; whereas
--metadata requires a newer server.

Although we don't use --security-info yet, the flag is already
documented for other dumpxml operations, and turning it on now
will make it useful when a future patch actually has to honor it.

* tools/virsh.c (cmdSnapshotDumpXML, cmdSnapshotCurrent): Add
--security-info.
(cmdSnapshotList): Add --roots, --metadata.
* tools/virsh.pod (snapshot-dumpxml, snapshot-current)
(snapshot-list): Document these.

To make it easier to know when undefine will fail because of existing
snapshot metadata, we need to know how many snapshots have metadata.

Also, it is handy to filter the list of snapshots to just those that
have no parents; document that flag now, but implement it in later patches.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_SNAPSHOT_LIST_ROOTS)
(VIR_DOMAIN_SNAPSHOT_LIST_METADATA): New flags.
* src/libvirt.c (virDomainSnapshotNum)
(virDomainSnapshotListNames): Document them.
* src/esx/esx_driver.c (esxDomainSnapshotNum)
(esxDomainSnapshotListNames): Implement trivial flag.
* src/vbox/vbox_tmpl.c (vboxDomainSnapshotNum)
(vboxDomainSnapshotListNames): Likewise.
* src/qemu/qemu_driver.c (qemuDomainSnapshotNum)
(qemuDomainSnapshotListNames): Likewise.

Adding this was trivial compared to the previous patch for fixing
qemu snapshot deletion in the first place.

* src/qemu/qemu_driver.c (qemuDomainSnapshotDiscard): Add
parameter.
(qemuDomainSnapshotDiscardDescendant, qemuDomainSnapshotDelete):
Update callers.

A future patch will make it impossible to remove a domain if it
would leave behind any libvirt-tracked metadata about snapshots,
since stale metadata interferes with a new domain by the same name.
But requiring snaphot contents to be deleted before removing a
domain is harsh; with qemu, qemu-img can still make use of the
contents after the libvirt domain is gone.  Therefore, we need
an option to get rid of libvirt tracking information, but not
the actual contents.  For hypervisors that do not track any
metadata in libvirt, the implementation is trivial; all remaining
hypervisors (really, just qemu) will be dealt with separately.

* include/libvirt/libvirt.h.in
(VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY): New flag.
* src/libvirt.c (virDomainSnapshotDelete): Document it.
* src/esx/esx_driver.c (esxDomainSnapshotDelete): Trivially
supported when there is no libvirt metadata.
* src/vbox/vbox_tmpl.c (vboxDomainSnapshotDelete): Likewise.

Similar to the last patch in isolating the filtering from the
client actions, so that clients don't have to reinvent the
filtering.

* src/conf/domain_conf.h (virDomainSnapshotForEachChild): New
prototype.
* src/libvirt_private.syms (domain_conf.h): Export it.
* src/conf/domain_conf.c (virDomainSnapshotActOnChild)
(virDomainSnapshotForEachChild): New functions.
(virDomainSnapshotCountChildren): Delete.
(virDomainSnapshotHasChildren): Simplify.
* src/qemu/qemu_driver.c (qemuDomainSnapshotReparentChildren)
(qemuDomainSnapshotDelete): Likewise.

Deleting a snapshot and all its descendants had problems with
tracking the current snapshot.  The deletion does not necessarily
proceed in depth-first order, so a parent could be deleted
before a child, wreaking havoc on passing the notion of the
current snapshot to the parent.  Furthermore, even if traversal
were depth-first, doing multiple file writes to pass current up
the chain one snapshot at a time is wasteful, comparing to a
single update to the current snapshot at the end of the algorithm.

* src/qemu/qemu_driver.c (snap_remove): Add field.
(qemuDomainSnapshotDiscard): Add parameter.
(qemuDomainSnapshotDiscardDescendant): Adjust accordingly.
(qemuDomainSnapshotDelete): Properly reset current.

This one's nasty.  Ever since we fixed virHashForEach to prevent
nested hash iterations for safety reasons (commit fba550f6),
virDomainSnapshotDelete with VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN
has been broken for qemu: it deletes children, while leaving
grandchildren intact but pointing to a no-longer-present parent.
But even before then, the code would often appear to succeed to
clean up grandchildren, but risked memory corruption if you have
a large and deep hierarchy of snapshots.

For acting on just children, a single virHashForEach is sufficient.
But for acting on an entire subtree, it requires iteration; and
since we declared recursion as invalid, we have to switch to a
while loop.  Doing this correctly requires quite a bit of overhaul,
so I added a new helper function to isolate the algorithm from the
actions, so that callers do not have to reinvent the iteration.

Note that this _still_ does not handle CHILDREN correctly if one
of the children is the current snapshot; that will be next.

* src/conf/domain_conf.h (_virDomainSnapshotDef): Add mark.
(virDomainSnapshotForEachDescendant): New prototype.
* src/libvirt_private.syms (domain_conf.h): Export it.
* src/conf/domain_conf.c (virDomainSnapshotMarkDescendant)
(virDomainSnapshotActOnDescendant)
(virDomainSnapshotForEachDescendant): New functions.
* src/qemu/qemu_driver.c (qemuDomainSnapshotDiscardChildren):
Replace...
(qemuDomainSnapshotDiscardDescenent): ...with callback that
doesn't nest hash traversal.
(qemuDomainSnapshotDelete): Use new function.

Each snapshot lookup was iterating over the entire hash table, O(n),
instead of honing in directly on the hash key, amortized O(1).

Besides, fixing this means that virDomainSnapshotFindByName can now
be used inside another virHashForeach iteration (without this patch,
attempts to lookup a snapshot by name during a hash iteration will
fail due to nested iteration).

* src/conf/domain_conf.c (virDomainSnapshotFindByName): Simplify.
(virDomainSnapshotObjListSearchName): Delete unused function.

Even though I recently added 'virsh snapshot-parent', doing it one
snapshot at a time is painful, so make it possible to expand the
snapshot-list table at once.

* tools/virsh.c (cmdSnapshotList): Add --parent.
* tools/virsh.pod (snapshot-list): Document it.

For a system checkpoint of a running or paused domain, it's fairly
easy to honor new flags for altering which state to use after the
revert.  For an inactive snapshot, the revert has to be done while
there is no qemu process, so do back-to-back transitions; this also
lets us revert to inactive snapshots even for transient domains.

* src/qemu/qemu_driver.c (qemuDomainRevertToSnapshot): Support new
flags.

Commit 5e47785b broke reverts to offline system checkpoint snapshots
with older qemu, since there is no longer any code path to use
qemu -loadvm on next boot.  Meanwhile, reverts to offline system
checkpoints have been broken for newer qemu, both before and
after that commit, since -loadvm no longer works to revert to
disk state without accompanying vm state.  Fix both of these by
using qemu-img to revert disk state.

Meanwhile, consolidate the (now 3) clients of a qemu-img iteration
over all disks of a VM into one function, so that any future
algorithmic fixes to the FIXMEs in that function after partial
loop iterations are dealt with at once.  That does mean that this
patch doesn't handle partial reverts very well, but we're not
making the situation any worse in this patch.

* src/qemu/qemu_driver.c (qemuDomainRevertToSnapshot): Use
qemu-img rather than 'qemu -loadvm' to revert to offline snapshot.
(qemuDomainSnapshotRevertInactive): New helper.
(qemuDomainSnapshotCreateInactive): Factor guts...
(qemuDomainSnapshotForEachQcow2): ...into new helper.
(qemuDomainSnapshotDiscard): Use it.