Comment # 3 on bug 1175822 from Franck Bui

(In reply to Fabian Vogt from comment #0)
> Is there anything that can be done to make udev triggered units stay across
> isolate?
> Maybe spice-vdagentd.socket needs IgnoreOnIsolate=true?

That would be the first option that comes to my mind.

Otherwise simply don't try to be too smart and start the socket unit
unconditionally.

That said, this is rather a design issue that should be discussed with
upstreams, so I'm closing this bug.