Regression: by default qemu opens both vnc and stdout console
Description of problem:
Running qemu with a vnc display (by default I'm not using the -display option) and -monitor stdio,
it fails because the display also wants the std output (it fails even if a pass the -vnc option).
If I remove the monitor I have both the vnc and the std output console at the same time.
I was able to use `-monitor stdio`, passing `-serial telent:...`
Steps to reproduce:
1. ./configure --enable-slirp --target-list=x86_64-softmmu --disable-user --disable-docs
2. make -j 4
3. qemu-system-x86_64 ... (without `-display` as shown above)
Additional information:
After bisecting I found the following commit changed the behavior:

```
commit 1bec1cc0da497e55c16e2a7b50f94cdb2a02197f
Author: Marc-André Lureau <marcandre.lureau@redhat.com>
Date:   Tue Sep 5 23:18:08 2023 +0400

    ui/console: allow to override the default VC

    If a display is backed by a specialized VC, allow to override the
    default "vc:80Cx24C".

    As suggested by Paolo, if the display doesn't implement a VC (get_vc()
    returns NULL), use a fallback that will use a muxed console on stdio.

    This changes the behaviour of "qemu -display none", to create a muxed
    serial/monitor by default (on TTY & not daemonized).

    Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Reviewed-by: Thomas Huth <thuth@redhat.com>
```