utils.ListVisibleFiles: Hide “lost+found” directories

If a “lost+found” directory is found at a filesystem's root path it is
ignored. In all other cases directory entries named “lost+found” are
treated normally. Unittests are included. Fixes issue 153.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>