core: Removing the use of getDisks from RestoreCommand

There is no place that initializes the disks in the
parameter class when RestoreAllSnapshot is called.
Since there is no use of it whatsoever, this patch removes the use of
it in the command, so we can decouple RestoreAllSnapshotsParameters from
TryBackToAllSnapshotsOfVmParameters

Change-Id: I1d0f5f92288ea92c114526e0d7d567a7ece45fbd
Bug-Url: https://bugzilla.redhat.com/867339
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>