core: introduce RemoveDiskSnapshotsCommand

* A new command (and task handler) for removing specific images
  of a disk (i.e. removing disks from VM snapshots).
* The command accepts as an argument a list of multiple images
  (of the same disk) to remove.
* The command utilizes SEAT infrastructure for removing multiple images
  from a disk consecutively (as merging multiple snapshots of a specific
  disk must be done separably - to avoid volume chain breakage).

Change-Id: Ia714a4390d1d9b672005be30f58b7fa98b9a31cd
Feature-Page: http://www.ovirt.org/Features/Snapshots_Overview
Signed-off-by: Daniel Erez <derez@redhat.com>