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>
Add some constants to the network model for drivers to use

Following on from

  commit 85f857e62a5edacbf467c19add6e51565ae3669f
  Author: Gary Kotton <gkotton@redhat.com>
  Date:   Sat Dec 8 08:54:16 2012 +0000

    Add vif_type to the VIF model

add some constants to nova.network.model to enumerate the valid
values for 'vif_type'. Also add a constant for max bridge name
length

Blueprint: libvirt-vif-driver
Change-Id: I47949c06fee732681bf0b547577f22a319d4e408
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Make nova network manager fill in vif_type

Make the Nova network manager impl fill in the 'vif_type' parameter
in the VIF configuration for instances. This is hardcoded to
VIF_TYPE_BRIDGE, since that's all Nova network supports.

Blueprint: libvirt-vif-driver
Change-Id: I46d1f0edc820ccc198e5a5c876ea25644ebedd86
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Move libvirt VIF XML config into designer.py"
Merge "Add some constants to the network model for drivers to use"
Merge "Make nova network manager fill in vif_type"