engine: Implement HostNetworkQos [de]serialization

Implemented HostNetworkQosMapper, which serializes and deserializes
HostNetworkQos entities according to the vdsm contract.

Change-Id: I8fab9e53938823463ff01af0a153390804f276c3
Signed-off-by: Lior Vernia <lvernia@redhat.com>