WITH_INTERFACE is not defined, it should be WITH_NETCF there to load
the interface driver.
Eric posted patch weeks ago to resolve the problems in the whole
build system, but it's not finalised yet:
https://www.redhat.com/archives/libvir-list/2012-June/msg01299.html
I'm going to simply fix the wrong macro name here so that the
interface driver could loaded, and continue the work on the listing
API for interface driver.