Modern day OpenBSD has qemu-ga as a package on its own. This introduces that:
https://undeadly.org/cgi?action=article;sid=20200514073852
But there has been more work since. It still revolves around this simple conf tweak:
agent: 1,type=isa
$ cat /etc/qemu/qemu-ga.conf
[general]
daemonize...