pipe: don't create a tranport descriptor for inhereted pipes

Otherwise we can get an error like this:
1: \t\tCreate transport fd /crtools-fd-1-5
...
1: Found id pipe:[122747] (fd 8) in inherit fd list
1: File pipe:[122747] will be restored from fd 9 duped from inherit

1: Error (util.c:131): fd 5 already in use (called at files.c:872)
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>