core: ReplicateDisk - prevent rollback on engine restart

VmReplicateDiskFinishTaskHandler:
Rollback should be prevented on VmReplicateDiskFinish success,
but we can't rely on the command's returnValue since
it's not persistent. Instead, to make sure rollback
is prevented on restart, query the DB to verify that
the disk indeed moved to the target storage domain.

Change-Id: Ibd088940d6e3f7be79fb0381076499ea1c1d73c7
Bug-Url: https://bugzilla.redhat.com/958848
Signed-off-by: Daniel Erez <derez@redhat.com>