Move virNetDevReplaceMacAddress and virNetDevRestoreMacAddress
to the virnetdev.c file where they naturally belong

* util/interface.c, util/interface.h: Remove
  virNetDevReplaceMacAddress and virNetDevRestoreMacAddress
* util/virnetdev.c, util/virnetdev.h: Add
  virNetDevReplaceMacAddress and virNetDevRestoreMacAddress