core: Add QoS to to DB and entities

Add the QoS entity to the DB, and backend (business entities, queries,
commands, messages and logs)

see: http://wiki.ovirt.org/Features/Design/Network_QoS

Change-Id: If39d20b77d06165e4adcc27e6b6dc5458cac93d3
Signed-off-by: Ofri Masad <omasad@redhat.com>