engine: Extracted Network QoS VDSM mapping logic

Extracted the code mapping a NetworkQoS entity to VDSM-readable format
to its own class NetworkQosMapper, to be shared with Host Network
QoS. This class is now able to also deserialize the VDSM format to
construct a NetworkQoS entity.

Change-Id: Ieae520848495091532993b95f938ed6d5ad50834
Signed-off-by: Lior Vernia <lvernia@redhat.com>