core: introduce customized snapshot preview support

* Updated the commands used for preview/commit/undo snapshots
  to support customized snapshots (by identifying and handling
  a custom disks list)
* SnapshotManager:
** generateVmConfiguration method now also accepts an argument for
   specified disks list (needed for custom preview).

Feature: http://www.ovirt.org/Features/Single_Disk_Snapshot

Change-Id: I89332a1538c7259dd3ea62d693ac062f47e52cd0
Signed-off-by: Daniel Erez <derez@redhat.com>