core: Improve performance of getdisksvmguid

getdisksvmguid is using view all_disks_including_snapshots which used
UNION instead of UNION ALL. This caused a needless performance hit.
On the performance environment it reduced execution from 2.2s to 700ms.

Change-Id: If037bf4a20f7cae9dd6ab6e7f95bcf0681b3bda1
Bug-Url: https://bugzilla.redhat.com/1141543
Signed-off-by: lzelkha@redhat.com <lzelkha@redhat.com>