I was able to root cause this, and with a small kernel patch, the PV drivers attach now. I've submitted the patch to OpenBSD for review (https://marc.info/?l=openbsd-tech&m=173154320510806&w=2).

One a side note, if anyone is reading this and knows how to "hide" the 9pfs device from a VM, such that the VM doesn't try to probe the device and load a driver for it, that would also likely solve this issue for OpenBSD VMs without needing a kernel patch.

.joel