Use default libvirt_vif_driver driver.

In Grizzly all the libvirt vif drivers were merged together.  Now
libvirt_vif_driver defaults to
  libvirt_vif_driver=nova.virt.libvirt.vif.LibvirtGenericVIFDriver
The only, non-deprecated driver.

https://wiki.openstack.org/wiki/LibvirtVIFDrivers
https://blueprints.launchpad.net/nova/+spec/libvirt-vif-driver

Fixes bug 1183557

Change-Id: I257931e086faef2a608e09a8b2facadc0699941a
Merge "Use default libvirt_vif_driver driver."