Disable netcf if building without libvirtd

Although the netcf interface driver can in theory be used by
the stateless drivers, in practice none of them want to use
it because they have different ways of dealing with interfaces.

Furthermore, if you have mingw32-netcf installed, then the
libvirt mingw32 build will fail with

  ../../src/interface/netcf_driver.c:644:5: error: unknown field 'close_used_without_including_unistd_h' specified in initializer

 * configure.ac: disable netcf if built without libvirtd

configure.ac

@@ -1613,6 +1613,9 @@ AC_ARG_WITH([netcf],
 
 NETCF_CFLAGS=
 NETCF_LIBS=
+if test "$with_libvirtd" = "no" ; then
+  with_netcf=no
+fi
 if test "$with_netcf" = "yes" || test "$with_netcf" = "check"; then
   PKG_CHECK_MODULES(NETCF, netcf >= $NETCF_REQUIRED,
     [with_netcf=yes], [