(please CC me as I'm not subscribed to the list)
I verified the behaviour on two different machines, both running archlinux
with QEMU 3.0.0 and reported it on https://bugs.archlinux.org/task/60141
where it was suggested I bring it to the list.
So what happens: if you try to run OpenBSD with VirtIO networking enabled,
the moment OpenBSD brings up network the QEMU process crashes. Easiest
steps to reproduce:
Download for example
https://ftp.halifax.rwth-aachen.de/pub/OpenBSD/6.3/amd64/install63.iso
Create new VM under libvirt as OpenBSD 6.3, create disk, "change options
before installation", change disk and network types to virtio
Proceed with installation (basically enter-enter-enter-...)
Observe qemu crash when network is brought up.
Sorry, I don't really know how to proceed further with that.