core: LiveMigrateVmDisksCommand - disk id for perm subject

Passing diskId instead of imageId for permission subject
(as done in I87d7733174b7b5af805f53834c64e032aafbe94b).

Change-Id: If10daec3924e99fb728ae65da0f5fbede07aed85
Bug-Url: https://bugzilla.redhat.com/1071687
Signed-off-by: Daniel Erez <derez@redhat.com>