Revert "core: Improve performance of getdisksvmguid"

This reverts commit d9e489471aaf0113fac7dff63845dee6f7f4523e.

UNION removes duplicate records, UNION ALL does not.
Hence, 'all_disks_including_snapshots' view includes duplicate
records for copied template disks.
Performance improvement should probably be achieved in another way.
In the mean time, reverting this change.

Change-Id: I5f6a0859a6000124e7968e5725ed7e5232b1a5cf
Bug-Url: https://bugzilla.redhat.com/1161612
Signed-off-by: Daniel Erez <derez@redhat.com>