When the block job event was first added, it was for block pull,
where the active layer of the disk remains the same name.  It was
also in a day where we only cared about local files, and so we
always had a canonical absolute file name.  But two things have
changed since then: we now have network disks, where determining
a single absolute string does not really make sense; and we have
two-phase jobs (copy and active commit) where the name of the
active layer changes between the first event (ready, on the old
name) and second (complete, on the pivoted name).

Adam Litke reported that having an unstable string between events
makes life harder for clients.  Furthermore, all of our API that
operate on a particular disk of a domain accept multiple strings:
not only the absolute name of the active layer, but also the
destination device name (such as 'vda').  As this latter name is
stable, even for network sources, it serves as a better string
to supply in block job events.

But backwards-compatibility demands that we should not change the
name handed to users unless they explicitly request it.  Therefore,
this patch adds a new event, BLOCK_JOB_2 (alas, I couldn't think of
any nicer name - but at least Migrate2 and Migrate3 are precedent
for a number suffix).  We must double up on emitting both old-style
and new-style events according to what clients have registered for
(see also how IOError and IOErrorReason emits double events, but
there the difference was a larger struct rather than changed
meaning of one of the struct members).

Unfortunately, adding a new event isn't something that can easily
be broken into pieces, so the commit is rather large.

* include/libvirt/libvirt.h.in (virDomainEventID): Add a new id
for VIR_DOMAIN_EVENT_ID_BLOCK_JOB_2.
(virConnectDomainEventBlockJobCallback): Document new semantics.
* src/conf/domain_event.c (_virDomainEventBlockJob): Rename field,
to ensure we catch all clients.
(virDomainEventBlockJobNew): Add parameter.
(virDomainEventBlockJobDispose)
(virDomainEventBlockJobNewFromObj)
(virDomainEventBlockJobNewFromDom)
(virDomainEventDispatchDefaultFunc): Adjust clients.
(virDomainEventBlockJob2NewFromObj)
(virDomainEventBlockJob2NewFromDom): New functions.
* src/conf/domain_event.h: Add new prototypes.
* src/libvirt_private.syms (domain_event.h): Export new functions.
* src/qemu/qemu_driver.c (qemuDomainBlockJobImpl): Generate two
different events.
* src/qemu/qemu_process.c (qemuProcessHandleBlockJob): Likewise.
* src/remote/remote_protocol.x
(remote_domain_event_block_job_2_msg): New struct.
(REMOTE_PROC_DOMAIN_EVENT_BLOCK_JOB_2): New RPC.
* src/remote/remote_driver.c
(remoteDomainBuildEventBlockJob2): New handler.
(remoteEvents): Register new event.
* daemon/remote.c (remoteRelayDomainEventBlockJob2): New handler.
(domainEventCallbacks): Register new event.
* tools/virsh-domain.c (vshEventCallbacks): Likewise.
(vshEventBlockJobPrint): Adjust client.
* src/remote_protocol-structs: Regenerate.
Signed-off-by: NEric Blake <eblake@redhat.com>