Libvirt driver to use current nova.network.model

* Updated tests to use VIF class instead of legacy "network,mapping" tuple.
* Modified libvirt/{driver,firewall,vif}.py to use VIF class instead of legacy
  "network,mapping" tuple.

Fixes: bug 1112646
Change-Id: Ic67291f3bfa7624af38061fa61a88ff16aa5acda