rest: delete a disk from disks collection of a snapshot

Introduce a support for deleting a specific disk from a snapshot:

DELETE /api/vms/{vm_id}/snapshots/{snapshot_id}/disks/{disk_id}

Change-Id: Ic7fb780516ba715d87e59e98651e9d03dad15845
Feature-Page: http://www.ovirt.org/Features/Snapshots_Overview
Signed-off-by: Daniel Erez <derez@redhat.com>