extra/virtual-stb: Add debug logging of netstat output

Really useful, since this is failing quite a lot.
extra/virtual-stb: Try again if X11 socket exists without PID

It *seems* that sometimes netstat reports this:

    [...]    -                   /tmp/.X11-unix/X11
    [...]    -                   @/tmp/.X11-unix/X11

instead of:

    [...]    8271/Xorg           /tmp/.X11-unix/X11
    [...]    8271/Xorg           @/tmp/.X11-unix/X11

This results in the following exception:

      File "extra/virtual-stb/virtual-stb.py", line 109, in start_x
        s = await_xorg_startup(xorg, display_no)
      File "extra/virtual-stb/virtual-stb.py", line 77, in await_xorg_startup
        p = xorg_pid(display_no)
      File "extra/virtual-stb/virtual-stb.py", line 62, in xorg_pid
        return int(pid)
    TypeError: int() argument must be a string or a number, not 'NoneType'

Maybe waiting a bit longer and running `netstat` again will give the
desired results.