Based exclusively on work by Eric Blake in a patch posted with the same
subject. However some modifications related to comments and my plans to
add another backend.

Added WITH_INTERFACE as the only automake variable deciding whether to
build the driver and using WITH_NETCF to identify that we're wanting to
use the netcf library as the backend.

* configure.ac: Added with_interface
* src/interface/netcf_driver.c: Renamed..
* src/interface/interface_backend_netcf.c: ..to this to match storage.
* src/interface/netcf_driver.h: Renamed..
* src/interface/interface_driver.h: ..to this.
* daemon/Makefile.am: Respect WITH_INTERFACE and WITH_NETCF.
* libvirt.spec.in: Add RPM support for --with-interface