engine : Null Pointer Exception for action CreateAllSnapshotsFromVm after failure in create snapshot

After a failure in create snapshot, engine enters into
a loop with NullPointerException. This patch fixes the
case where the vdsm was restarted after submission of the
command and the entry in snapshots table was cleared by engine
leading to NPE in CreateAllSnapshotsFromVmCommand.

Change-Id: If89c45a236029078eae4a4254837b373c88adfd2
Bug-Url: https://bugzilla.redhat.com/982003
Signed-off-by: Ravi Nori <rnori@redhat.com>