virtual-input-device/native: Emit added/removed events

Without these devices, things that depend on the existance of input
device classes won't know about the existance of e.g. pointer devices,
if the only pointer device is from a virtual one.

This requires handling situations where e.g. a device doesn't have a
device node thus can't be matched against a udev device.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1688>

src/backends/meta-input-mapper.c

@@ -386,6 +386,9 @@ input_device_get_physical_size (MetaInputMapper    *mapper,
   const char *node;
 
   node = clutter_input_device_get_device_node (device);
+  if (!node)
+    return FALSE;
+
   udev_device = g_udev_client_query_by_device_file (mapper->udev_client, node);
 
   if (udev_device &&