QEMU most likely is not required. OpenBSD's installer is inside a single 4.5 MiB [1] ramdisk kernel image. Chainload or netboot it, or download the ramdisk to ffs on sd0 and run installboot [2]. Once the ramdisk kernel is loaded you can erase the disk containing it.
1. https://cdn.openbsd.org/pub/OpenBSD/7.5/amd64/bsd.rd
2. installboot also needs /usr/mdec/biosboot and /usr/mdec/boot from base75.tgz.