core: GetRngDeviceQuery should be a user query

Change-Id: I5589c7185a61c076123aee962c8ea2c097920617
Bug-Url: https://bugzilla.redhat.com/1122427
Signed-off-by: Oved Ourfali <oourfali@redhat.com>
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>