core: Add CRUD commands and queries for QoS objects (storage)

For more information see: http://www.ovirt.org/Features/aggregate_QoS,
    http://www.ovirt.org/Features/blkio-support

Change-Id: I1a9af59277b5055453159f002f19046c00522ddb
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>