libvirt: remove pointless LibvirtBaseVIFDriver class

There is only one single VIF driver class these days, so it is
pointless having a LibvirtBaseVIFDriver base class to inherit
from.

Related-bug: #1302796
Change-Id: Ie132135f9b6cf5337782ee9d1611c05bd56557fe
Merge "libvirt: remove pointless LibvirtBaseVIFDriver class"