tty: Update tty verification procedure

We need to make sure that the only one external tty
is present, since otherwise we have no idea on which
pts index to connect them on restore.

This patch extends tty_verify_active_pairs procedure to
count how many external tty is present in the dump image.

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