https://bugzilla.redhat.com/show_bug.cgi?id=1600330

Add nwfilterbinding schema in virt-xml-validate for autoprobing.
Add document of nwfilterbinding schema in tools/virt-xml-validate.pod.
Signed-off-by: NHan Han <hhan@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>