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
Related-To: https://bugzilla.redhat.com/1085395
Signed-off-by: Tal Nisan <tnisan@redhat.com>