We already print availabled devices with "-device help", or available
backends with "-netdev help" or "-chardev help". Let's provide a way
for the users to query the available display backends, too.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 20200108144702.29969-1-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>