Commit c388f408 added the possibility to list the display
backends using '-display help'. Since the 'none' backend is
is not implemented as a DisplayChangeListenerOps, it is not
registered to the dpys[] array with qemu_display_register(),
and is not listed in the help output.

This might be confusing, as we list it in the man page:

  -display type
      Select type of display to use. This option is a replacement for
      the old style -sdl/-curses/... options. Valid values for type are

      none
          Do not display video output. The guest will still see an
          emulated graphics card, but its output will not be displayed
          to the QEMU user. This option differs from the -nographic
          option in that it only affects what is done with video
          output; -nographic also changes the destination of the serial
          and parallel port data.

Fix by manually listing the special 'none' backend in the help.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20200120192947.31613-1-philmd@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>