core: getdisksvmguid hit the perf due to all_disks_including_snapshots view

Add index
Replace UNION with UNION ALL, which doing group by of the images
to avoid duplicates

Change-Id: I076e804cab601f3873cf2fdccba77abe42da60d3
Bug-Url: https://bugzilla.redhat.com/1141543
Signed-off-by: lzelkha@redhat.com <lzelkha@redhat.com>