It’s given access to an unformatted zvol that was created specifically for the VM, so that wouldn’t be the problem.
bhyve takes disks via AHCI, which is what it’s always done, and that worked just fine with Windows 7 previously. I’m pretty sure that that’s how my actual hardware is set up as well, and it installs Windows 7 just fine, since I set that up to dual boot.
Given that this worked previously, it seems likely that something with bhyve changed since the last time I did this. And given the fact that it I’ve had FreeBSD and Linux VMs suddenly start failing to boot, with the bhyve UEFI stuff not loading their bootloaders, I suspect that either bhyve has some serious problems at the moment (at least with what I currently have set up), or my machine has issues of some kind that it didn’t have before. bhyve seems incredibly flaky for me right now in a way that it wasn’t at the end of last year.