block/parallels: implement parallels_check method of block driver

The check is very simple at the moment. It calculates necessary stats
and fix only the following errors:
- space leak at the end of the image. This would happens due to
  preallocation
- clusters outside the image are zeroed. Nothing else could be done here

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Roman Kagan <rkagan@parallels.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Roman Kagan <rkagan@parallels.com>
Message-id: 1430207220-24458-20-git-send-email-den@openvz.org
CC: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
