ui/console: simplify getting active_console size

We can get the active console dimension regardless of its kind, by
simply giving NULL as argument. It will fallback with the given value
when the dimensions aren't known.

This will also allow to move the code in a separate unit more easily.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230830093843.3531473-33-marcandre.lureau@redhat.com>