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
Merge "virt: delete unused 'interface_stats' method"