DBusDisplayConsole is specific to a given QemuConsole.

Fixes: commit 142ca628 ("ui: add a D-Bus display backend")
Reported-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>