Rename the ifaceCheck method to virNetDevValidateConfig and change
so that it always raises an error and returns -1 on error.

* src/util/interface.c, src/util/interface.h: Rename ifaceCheck
  to virNetDevValidateConfig
* src/nwfilter/nwfilter_gentech_driver.c,
  src/nwfilter/nwfilter_learnipaddr.c: Update for API rename