core, db: add getQosByDiskProfile

a single query to avoid 2:
* getDiskProfile
* getQos

Change-Id: If8e9b246172497412b3dc13d46af85fd155713cf
Bug-Url: https://bugzilla.redhat.com/1085049
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>