Update coding guidelines on guest_arch

[jstarks]

That's a very insightful comment John, thank you. Thinking about it in terms of worker code vs. "client" code is not something I had considered, but makes a lot of sense.

Could you send a PR up which adds these thoughts to https://openvmm.dev/guide/dev_guide/contrib/code.html, so we can codify some of this thinking? Or, if you'd like to drive-by include it as part of this PR (which looks like it'll need at least one more revision anyways), that'd be great.

Originally posted by @daprilik in #307 (comment)