core: disk migrate - avoid ctor init on last task handler

LiveMigrateDiskCommand - avoid initialization on constructor
in case of last task handler:
* No need to fetch un-needed stale data after last task completion.
* Prevent an NPE on getVm() -> getVmsListForDisk()
  [as getImageGroupId() can be null in case of deleting the source
  disk before the last task (deleteImage) has been completed].

Change-Id: I4c8f14fdcc5e7db3cca41d2ce2e37e97a034ca23
Bug-Url: https://bugzilla.redhat.com/971022
Signed-off-by: Daniel Erez <derez@redhat.com>