libvirt: remove 3 unused vif.py methods

In the following commit:

  commit 842b2abfe76dede55b3b61ebaad5a90c356c5ace
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Mon Nov 25 14:00:25 2013 +0000

    Increase min required libvirt to 0.9.11

support for setting up OVS outside of libvirt was dropped,
but 3 of the helper methods were mistakenly not deleted.

Related-bug: #1254727
Change-Id: I53de8703a64d1214ec81b9698e376737a92cb53a
Merge "libvirt: remove 3 unused vif.py methods"