util: add virCommandPassListenFDs() function
    
    That sets a new flag, but that flag does mean the child will get
    LISTEN_FDS and LISTEN_PID environment variables properly set and
    passed FDs reordered so that it corresponds with LISTEN_FDS (they must
    start right after STDERR_FILENO).
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>