vm: add known case for graceful destroy to fail

In commit 09c2f40ebc318bbd188696fd7d02d2dc73d03256
we changed the releaseVm flow to be more strict and do not swallow libvirt
exceptions when domain destruction fails. In these cases the destroy()
API call will now fail.

Unfortunately, one particular case escaped verification, and broke.
When a migration succesfully ends, libvirt notifies a 'Stopped' event,
with detail VIR_DOMAIN_EVENT_STOPPED_MIGRATED.
In response to this, VDSM correctly puts the VM as Down, but does not
trigger the internal onQemuDeath callback, which will invalidate the
domain handle (Vm._dom field).

The sequence of events is further demonstrated by this excerpt
(Logs trimmed for brevity):

vm.Vm::(_onLibvirtLifecycleEvent)
vmId=`56d1c657-dd76-4609-a207-c050699be5be`::event Stopped detail 3
opaque None
vm.Vm::(cancel) vmId=`56d1c657-dd76-4609-a207-c050699be5be`::canceling
migration downtime thread
vm.Vm::(stop) vmId=`56d1c657-dd76-4609-a207-c050699be5be`::stopping
migration monitor thread
vm.Vm::(run) vmId=`56d1c657-dd76-4609-a207-c050699be5be`::migration
downtime thread exiting
root::(wrapper) Unknown libvirterror: ecode: 42 edom: 10 level: 2
message: Domain not found: no domain with matching uuid
'56d1c657-dd76-4609-a207-c050699be5be'
root::(wrapper) Unknown libvirterror: ecode: 42 edom: 10 level: 2
message: Domain not found: no domain with matching uuid
'56d1c657-dd76-4609-a207-c050699be5be'
vm.Vm::(setDownStatus)
vmId=`56d1c657-dd76-4609-a207-c050699be5be`::Changed state to Down:
Migration succeeded (code=4)

Later, when Engine sends a Destroy command as part of the normal flow,
in releaseVm we will find a supposedly valid _dom handle (being not
None), and then the domain destruction will be triggred.
But being actually the handle stale, the call and will fail for
missing domain; then in turn the whole destroy() API call will fail.

It is worth to be noted that 09c2f40ebc318bbd188696fd7d02d2dc73d03256
only made evident that in this case VDSM was out of sync with libvirt.

This patch address this problem by adding a known benign case
on which the gracful destruction can fail.

Change-Id: I42bf1fd78e439988e0cc60258a51fa2bc447e0f1
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28952
Reviewed-by: Dan Kenigsberg <danken@redhat.com>