rest: retrieve disk snapshots list by storage domain

* Introduce a support for retrieving disk snapshots from
  storage domain context:

- GET /api/storagedomains/{storage_id}/disksnapshots
- Added a new rest entity - DiskSnapshot:
-- Represents a snapshot of a disk.
-- Its ID is the image_id; as DiskImage in backend
   composed of an image_group_id (the id of the Disk),
   and a list of DiskImages (snapshots) which differentiate
   by image_id. Hence, each DiskSnapshot can be uniquely
   identified by the image_id.
-- Contains a link to the container disk (image_group).

* Introduce a support for deleting a disk snapshot from
  storage domain:

- DELETE /api/storagedomains/{storage_id}/disksnapshots/{image_id}

Change-Id: I5359c5df490c7e7e6f9b4d25e794bd07c6877339
Feature-Page: http://www.ovirt.org/Features/Snapshots_Overview
Signed-off-by: Daniel Erez <derez@redhat.com>