virt: delete unused 'interface_stats' method

The only use of the virt driver 'interface_stats' method was
removed 3+ years ago by

  commit 07646e85841a4f7c81e80254ac63715bece2aadd
  Author: Brian Waldon <brian.waldon@rackspace.com>
  Date:   Tue Aug 2 10:09:58 2011 -0400

    removing compute monitor

Change-Id: Ie43ebea3ecabd0dd11a5c341bf2310a00f143031
virt: Convert more tests to use instance objects

Update a couple of methods in test_virt_drivers so that they
use instance objects instead of dicts

Blueprint: libvirt-driver-class-refactor
Change-Id: Ib2257ac97c533a8a0d7f45994d21a67d08444e19
Updated from global requirements

Change-Id: I0d572918d958bcff6ab15af9ccea09401e53ebac
Fix base obj_make_compatible() handling ListOfObjectsField

This makes the base class' obj_make_compatible() handle lists of
objects properly.

Related to blueprint kilo-objects

Change-Id: Iaff1b435819e9aefcd2b362c846c87f9efa3f618
Make objects use the generalized backport scheme

This makes the objects that have subobjects use the base class'
obj_make_compatible() implementation for backports.

Related to blueprint kilo-objects

Change-Id: I106cd57001f028de7cc8ce7c7024809bceaed020
Merge "Fix base obj_make_compatible() handling ListOfObjectsField"
Merge "Updated from global requirements"
Merge "virt: delete unused 'interface_stats' method"
Merge "virt: Convert more tests to use instance objects"
Merge "Make objects use the generalized backport scheme"