> OTOH, OpenBSD's VMD not only supports it but I think it's enabled by default.

Yes, my VMD virtual machines have it with no configuration necessary.

    pvbus0 at mainbus0: OpenBSD
    pci0 at mainbus0 bus 0
    virtio0 at pci0 dev 1 function 0 "Qumranet Virtio RNG" rev 0x00
    viornd0 at virtio0

Here’s the manpage: https://man.openbsd.org/viornd.4