libxl: Fix segfault in device_model_spawn_outcome

libxl__spawn_qdisk_backend() explicitly sets guest_config to NULL when
starting QEMU (the usual launch through libxl__spawn_local_dm() has a
guest_config though).

Bail early on a NULL guest_config/d_config.  This skips the QMP queries
for chardevs and VNC, but this xenpv QEMU instance isn't expected to
provide those - only qdisk (or 9pfs backends after an upcoming change).

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>

tools/libs/light/libxl_dm.c

@@ -3172,8 +3172,8 @@ static void device_model_spawn_outcome(libxl__egc *egc,
 
     /* Check if spawn failed */
     if (rc) goto out;
-
-    if (d_config->b_info.device_model_version
+    /* d_config is NULL for xl devd/libxl__spawn_qemu_xenpv_backend(). */
+    if (d_config && d_config->b_info.device_model_version
             == LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN) {
         rc = libxl__ev_time_register_rel(ao, &dmss->timeout,
                                          devise_model_postconfig_timeout,