VNC code relies on con==NULL to mean the default console.

Fixes:
https://gitlab.com/qemu-project/qemu/-/issues/1548



Fixes: commit 385ac97f ("ui: keep current cursor with QemuConsole")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reported-by: Helge Konetzka <hk@zapateado.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230319111017.1319880-1-marcandre.lureau@redhat.com>