tty: Make sure deprecated logic does not take wrong branch

Don't do additional processing for ext-tty which are
expected to not have regfile in image.

Reported-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>