nwfilter: wire up new APIs for creating and deleting nwfilter bindings

nwfilter: wire up new APIs for creating and deleting nwfilter bindings

This allows the virsh commands nwfilter-binding-create and
nwfilter-binding-delete to be used.

Note using these commands lets you delete filters that were
previously created automatically by the virt drivers, or add
filters for VM nics that were not there before. Generally it
is expected these new APIs will only be used by virt drivers.
It is the admin's responsibility to not shoot themselves in
the foot.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>