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.