zdtm: Add /dev/tty into construct_root and prepare_mntns

We will need it for tty testing.

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