Fix crash in QEMU auto-destroy with transient guests
Fix crash in QEMU auto-destroy with transient guests

https://bugzilla.redhat.com/show_bug.cgi?id=950286

When the auto-destroy callback runs it is supposed to return
NULL if the virDomainObjPtr is no longer valid. It was not
doing this for transient guests, so we tried to virObjectUnlock
a mutex which had been freed. This often led to a crash.

Signed-off-by: Daniel P. Berrange <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>
(cherry picked from commit 7ccad0b16d12d7616c7c21b1359f6a55a9677521)

mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2