The QemuTextConsole code paths assume a surface is being used as
scanout, let's make this more explicit.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230830093843.3531473-39-marcandre.lureau@redhat.com>