color-manager: Don't try to manage virtual monitors

There is no way to set any gamma luts, or do anything other color
management related. Eventually we'll probably want to, but that requires
bringing color management plumbing to PipeWire.

Doing this is also needed when running a headless session, as when
headless, polkit doesn't let us create colord devices without explicit
user permission, meaning we'll spam the session with useless dialogs
each time a session is started.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3423>

src/backends/meta-color-manager.c

@@ -185,6 +185,9 @@ update_devices (MetaColorManager *color_manager)
       g_autofree char *stolen_monitor_id = NULL;
       MetaColorDevice *color_device;
 
+      if (meta_monitor_is_virtual (monitor))
+        continue;
+
       monitor_id = generate_monitor_id (monitor);
 
       if (priv->devices &&