core: Replaced the check of whether a storage pool contains mixed type

Replaced the check of whether a storage pool contains storage domains of
mixed types to use a designated query that fetches distinct storage types
instead of fetching all the storage domains and filtering by that, in that
way we are fetching less data and eliminating the use for logic in engine
to determine if the domain is mixed thus resulting in better performance

Change-Id: I86cf1512655865831de3cbd16c10a7bc243f08cd
Signed-off-by: Tal Nisan <tnisan@redhat.com>