core: VmReplicateDiskFinish on SyncImageData failure

Requirements:
* VmReplicateDiskFinish should be sent on SyncImageData failure.
* Further rollback should be prevented on VmReplicateDiskFinish failure.
* Disk should always be unlocked after failure.

Implementation details:
* Added 'preventRollback' method to TaskHandlerCommand interface.
* Implemented 'preventRollback' on LiveMigrateDiskCommand.
* Called 'revertTask' on 'endWithFailure' to ensure revert on task failure.

Change-Id: I6c25b02cb0c2bd78b012dd3e3654caa43f5141d6
Bug-Url: https://bugzilla.redhat.com/961423
Signed-off-by: Daniel Erez <derez@redhat.com>