When migrating process it might not have slave tty peers at
all so instead of exiting early just wait for its real usage
and only then fail.
Reported-by: Manuel Rodríguez Pascual <manuel.rodriguez.pascual@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>