core: MoveOrCopyImageGroup - fixes and changes to revertTasks

currently the revertTasks method in MoveOrCopyImageGroupCommand issues a
RemoveImage command in order to attempt to revert to operation. The
issued RemoveImage command parameters are set with
MoveOrCopyImageGroupCommand as the parent command.

*Removes unneeded further execution of revertTasks method upon the
completion of each RemoveImage.
*In case of import vm failure, the disks will be removed during
RemoveImage endSuccesfully method.
*added execution of revert tasks in case of import vm.

Change-Id: I0f767fcfa05d48ba085910dac7a1b746601381a9
Bug-Url: https://bugzilla.redhat.com/890922
Signed-off-by: Liron Aravot <laravot@redhat.com>