[v8.0.4 regression] `qemu-system-x86_64: -accel hvf: Unknown Error`
Description of problem:
`-accel hvf` crashes with "Unknown Error".
Regression in v8.0.4.

The master branch doesn't seem affected.
Steps to reproduce:
v8.0.3:
```console
$ qemu-system-x86_64 -accel hvf
(shows iPXE screen, as expected)
```

v8.0.4:
```console
$ qemu-system-x86_64 -accel hvf
qemu-system-x86_64: -accel hvf: Unknown Error
Abort trap: 6
```
Additional information: