conf: change virNWFilterBindingPtr to virNWFilterBindingDefPtr
    
    We are going to want to expose the NWFilter binding concept in the
    public API, so the virNWFilterBindingPtr type needs to be used there.
    Our internal type will shortly gain an XML representation, so rename
    it to virNWFilterBindingDefPtr which follows our normal conventions.
    
    Reviewed-by: John Ferlan <jferlan@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>