> It's less about perf/security and more to do with simply needing the time to wrap my head around how to do it.

I can have a look at using ppp for creating a network connection using stdio. It doesn't look very complicated.

> it's hard to test kernel code

Exactly why I suggest pppd (userland) instead of VFIO

> So I not only need to build the kernel but I need to emulate the CPU features the kernel needs too

You have stdio? No need for anything else.