Quantum: use generic VIF driver

For plugins that support VIF bindings, make use of the
LibvirtGenericVIFDriver libvirt vif driver. The plugin
returns the VIF type to Nova which inturn "knows" what
to do.

Change-Id: I82982a9f1324db878439aa6bbee38bd205f073d0
Merge "Quantum: use generic VIF driver"