conf: move virNWFilterBindingDefPtr into its own files
    
    There's no code sharing between virNWFilterDef and
    virNWFilterBindingDefPtr types, so it is clearer if they live in
    separate source files and headers.
    
    Reviewed-by: John Ferlan <jferlan@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>