engine: Introduce HostNetworkInterfacesPersister

The class will be used to persist the network interfaces
as reported by VDSM to the engine db:
* New nics will be created
* Existing nics will be updated
* Unreported nics will be deleted

This class will later be used for persisting the network
attachments entities of each network interface.

Change-Id: Iec0701a302781a9fa147c65818764bddf8429828
Signed-off-by: Moti Asayag <masayag@redhat.com>