engine: SearchQuery generates slow query on vds_with_tags and storage_domains

Fix SearchQuery so that queries without tags are run against the
base table, saving an uneeded (and costly) inner query. VDS query decreased
by almost 80% (from 100ms to 15ms).

Change-Id: Ia0b7a4912d031c4cdafd636126730ea81d8d2bdd
Bug-Url: https://bugzilla.redhat.com/984973
Signed-off-by: lzelkha@redhat.com <lzelkha@redhat.com>