dal: Added a mixed storage pool to fixtures.xml

Added a mixed storage pool to fixtures.xml file to allow dao tests to test
cases of mixed storage pools also

Change-Id: I285c420902ce8d28cb7e8bfcc2ad3a11c225c0fb
Bug-Url: https://bugzilla.redhat.com/1085395
Signed-off-by: Tal Nisan <tnisan@redhat.com>
core: Added a query to fetch storage types in a storage pool

Added a query that fetches all the storage types of the data storage domains
contained in a certain pool

Change-Id: I97447e0fc3e087958101156947bded12c1fb9418
Bug-Url: https://bugzilla.redhat.com/1085395
Signed-off-by: Tal Nisan <tnisan@redhat.com>
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
Bug-Url: https://bugzilla.redhat.com/1083560
Signed-off-by: Tal Nisan <tnisan@redhat.com>