"chr_option_parsed" is only implemented by the "mux" chardev, we can
specialize the code there to avoid the needless generic class method.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>