files-reg: Don't generate ghost files for hanged out slave pty peers

We will use reg-files engine for tty c/r so lets prepare scaffolds
here preventing the engine to generate ghost files for PTY peers
(mostly because one can't create slave peers without opening
 master peers first which is a sole part of tty engine itself).

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>