Fix up the implementation of define_nwfilter_xml.
Fix up the implementation of define_nwfilter_xml.

It was using a totally bogus rb_scan_args call.  In any case,
it accepts one (and only one) argument, so just have ruby
enforce that for us.

Signed-off-by: Chris Lalancette <60b62644009db6b194cc0445b64e9b27bb26433a@redhat.com>

libvirt/ruby-libvirt,libvirt/ruby-libvirt,libvirt/ruby-libvirt