Delete pointless nova.virt.VIFDriver class

There is a nova.virt.VIFDriver class which the hypervisor
VIF drivers inherit from. None of the hypervisor subclasses
pay any attention to the API contract defined to the parent
class. Some of the hypervisors don't bother trying to use
the VIFDriver class at all (hyperv, baremetal).

It is not worth trying to make the hypervisor subclasses
follow the same API contract, since the base parent class
offers no re-usable functionality of its own. The hypervisor
base classs nova.virt.ComputeDriver also has no need for the
VIFDriver API to exist, since its integration point is the
'plug_vifs' method.

As such the nova.virt.VIFDriver class serves no purpose
other than to mislead reviewers into thinking there is a
common API where none actually exists. It should thus be
deleted

Change-Id: Id287f7e66143a248a5f106e010510a3f09d2b1e7
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Delete pointless nova.virt.VIFDriver class"