conf: implement NetworkObj backend of virNetworkUpdate API

conf: implement NetworkObj backend of virNetworkUpdate API

virNetworkObjUpdate takes care of all virNetworkUpdate-related changes
to the data stored in the in-memory virNetworkObj list. It should be
called by network drivers that use this in-memory list.

virNetworkObjUpdate *does not* take care of updating any disk-based
copies of the config, nor does it perform any other operations
necessary to have the new config data take effect (e.g. it won't
re-write dnsmasq host files, nor will it send a SIGHUP to dnsmasq) -
those things should all be taken care of in the network driver
function that calls virNetworkObjUpdate (assuming that it returns
success).