qemu-options: Rework the help text of the '-display' option

qemu-options: Rework the help text of the '-display' option

Improve the help text of the "-display" option:

- Only print the options that we have enabled in the binary
  (similar to what we do for other options like -netdev already)

- The "frame=on|off" from "-display sdl" has been removed in commit
  09bd7ba9f5f7 ("Remove deprecated -no-frame option"), so we should
  not show this in the help text anymore

- The "-display egl-headless" line was missing a "\n" at the end

- Indent the default display text in a nicer way

Signed-off-by: Thomas Huth <huth@tuxfamily.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20191023120129.13721-1-huth@tuxfamily.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>