Comment 7 for bug 1577001

I created https://github.com/systemd/systemd/issues/3171 for changing the socket group on reload with an absent unit. This will not be an issue any more when lxd's prerm stops `lxd.socket` on removal, but (1) this sounds worth fixing anyway, and (2) I have some feeling that the above reproducer is not the complete story here yet. Waiting on Dan's answer for that.