libxl: fix error message in local_device_attach_cb

The fields that are printed might not be set in the case of a failure, which
generates a segmentation fault.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/libxl/libxl.c

@@ -3209,10 +3209,7 @@ static void local_device_attach_cb(libxl__egc *egc, libxl__ao_device *aodev)
 
     rc = aodev->rc;
     if (rc) {
-        LOGE(ERROR, "unable to %s %s with id %u",
-                    libxl__device_action_to_string(aodev->action),
-                    libxl__device_kind_to_string(aodev->dev->kind),
-                    aodev->dev->devid);
+        LOGE(ERROR, "unable locally attach device: %s", disk->pdev_path);
         goto out;
     }