zdtm: Mount devpts in new root

In case if we enter the new rootfs don't forget to
mount devpts, otherwise ttys can't be connected.

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