Move libvirt VIF XML config into designer.py

There are a fixed set of sensible XML configurations for
network VIFs. Create some helper methods for configuring
these configurations. Then update the VIF drivers to call
these APIs. The 'designer.py' file will serve as a place
for any policy based XML configuration helpers, as distinct
from config.py which is purely dealing with XML parsing
and formatting

Blueprint: libvirt-vif-driver
Change-Id: I857607c15ed576c65f898df4eb7533505742324b
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Move libvirt VIF XML config into designer.py"