VmStatsThread: make _getNicStats testable

With this test, the shame of change Ie356d5f96c73b6 should not repeat.

Change-Id: Ibd6ffa12341b814775bea96a47365645b571f3a8
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25982
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>