Refactor nwfilter parameters

* libvirt < 1.0.1 does not allow dynamically updating filter
  parameters inside an interface block of theinstance XML. To
  address this we move the nwfilter paramters from the instance
  XML to the per-instance base nwfilter.

Relates to bug #1124722

Change-Id: I50ec5dcffa8ff0232ae0aba0a08fce5151962238
Merge "Refactor nwfilter parameters"