Remove deprecated libvirt VIF driver code

Libvirt vif specific driver classes have been replaced by a generic
driver LibvirtGenericVIFDriver and are deprecated in Havana, so we
can remove them safely.

DocImpact

Change-Id: Icecb3c9aa4282c511364107d87f8aba517a5ae14
Closes-Bug: 1235036
Merge "Remove deprecated libvirt VIF driver code"