libxl: allow to run non-HVM guest without any device model being available

When removing qemu-trad support, libxl__domain_build_info_setdefault()
was modified to only allow qemu-xen as a device model. This change
resulted in an error at domain creation when qemu-xen isn't there.

As this is a perfectly fine situation for PV and PVH guests without
any qemu based backends, handle this case gracefully.

Fixes: 19ab8356ab ("tools: remove support for running a guest with qemu-traditional")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>

tools/libs/light/libxl_create.c

@@ -105,8 +105,8 @@ int libxl__domain_build_info_setdefault(libxl__gc *gc,
         dm = libxl__domain_device_model(gc, b_info);
         rc = access(dm, X_OK);
         if (rc < 0) {
-            LOGE(ERROR, "qemu-xen access error");
-            return ERROR_FAIL;
+            LOGE(INFO, "qemu-xen is unavailable");
+            b_info->device_model_version = LIBXL_DEVICE_MODEL_VERSION_UNKNOWN;
         }
     }