qdev-monitor: print help to stdout

qdev_device_help() is used from command line "-device help", or from
HMP "device_add". If used from command line, print help to stdout
(it is only printed on explicit demand).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>