Defer BrlAPI tty acquisition to when guest starts using device

We do not want to catch the BrlAPI input/ouput immediately, but only
when the guest has started discussing withour virtual device.

This notably fixes input before the guest driver has started.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>