The ifaceUp, ifaceDown, ifaceCtrl & ifaceIsUp APIs can be replaced
with calls to virNetDevSetOnline and virNetDevIsOnline

* src/util/interface.c, src/util/interface.h: Delete ifaceUp,
  ifaceDown, ifaceCtrl & ifaceIsUp
* src/nwfilter/nwfilter_gentech_driver.c, src/util/macvtap.c:
  Update to use virNetDevSetOnline and virNetDevIsOnline