init: Add comment that we should consider using /proc/consoles.

Although we can't use it at the moment because it is incorrect
on at least s390x (RHBZ#1351968).

appliance/init

@@ -185,6 +185,7 @@ else
     sleep 2
 
     # Get name of the serial port, from console= passed by libguestfs.
+    # XXX Consider using /proc/consoles
     guestfs_serial=$(grep -Eo 'console=[^[:space:]]+' /proc/cmdline |
                      sed s/console=//)