libvirt: Make nwfilter driver use right filterref

This fixes Bug #1220856 which occurs with Libvirt NWFilterFirewall
Driver.  While creating a VM with multiple NICs including one connects
to DHCP serving network and another to DHCP non-serving network,
NWFilterFirewall Driver will use the same base filter that allows DHCP
for all NICs. This leads a libvirt launch error due to lack of
'dhcp_server' parameter which is needed to define allow-DHCP filters.

This patch makes NWFilterFirewall Driver use right base filter for
each NIC depends on 'dhcp_server' config.

Closes-bug: #1220856
Change-Id: I7f9a7c281f152985478b2ec295f0644ba475fd76
Merge "libvirt: Make nwfilter driver use right filterref"