LibvirtBridgeDriver now LibvirtGenericVIFDriver

fixes bug 1106441

In Grizzly, inheritance is such that LibvirtGenericVIFDriver does
the same job as LibvirtBridgeDriver, and there are long term (H/I)
plans to retire libvirtBridgeDriver. This patch updates the config
reference.

Change-Id: Ie571b86f8559218f0c7a54f273b56a24c4543afd
Merge "LibvirtBridgeDriver now LibvirtGenericVIFDriver"