core: Fix quota calculation for floating disks

Disk which are not connected to a VM are not counted when calculating
storage quota.

The DB script was updated to include also disks which are not connected
to a VM.

Change-Id: I0da0a2535897076c4512384e65c0aea5ce28a0df
Bug-Url: https://bugzilla.redhat.com/913408
Signed-off-by: Ofri Masad <omasad@redhat.com>