libxl: hotplug scripts: stdout >& stderr

Plumb hotplug scripts' stdout to stderr.  That way if they print
anything (which really they shouldn't), it won't get mixed up with
the application's stdout.  (Eg, perhaps with an xl migration
stream...)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Vasiliy Tolstov <v.tolstov@selfip.ru>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxl/libxl_device.c

@@ -1031,7 +1031,7 @@ static void device_hotplug(libxl__egc *egc, libxl__ao_device *aodev)
 
     if (!pid) {
         /* child */
-        libxl__exec(gc, -1, -1, -1, args[0], args, env);
+        libxl__exec(gc, -1, 2, -1, args[0], args, env);
         /* notreached */
         abort();
     }