core: GetAllDisksBySD - fetch disks snapshots

* GetAllDisksByStorageDomainIdQuery:
fetch disks' snapshots for calculating the actual size
(DiskImage -> getActualDiskWithSnapshotsSize).

* Remove redundant GetImagesByStorageId SP.

Change-Id: I055e317ea21a9eabea126e9ca17af61db659cd10
Bug-Url: https://bugzilla.redhat.com/905595
Signed-off-by: Daniel Erez <derez@redhat.com>