core: fix NPE when migration caused by maintenance fails

On failure to migrate VM from VDS which is going to maintenance, we got
NPE in ExecutionHandler while trying to end the job.

The solution is to pass the job from the MaintenanceVds command to the
InternalMigrateVm command it invokes as well.

Change-Id: If1bde265b078997507ad7f7e4b00fbd024cc3c39
Bug-Url: https://bugzilla.redhat.com/1053884
Signed-off-by: Arik Hadas <ahadas@redhat.com>