The unix-block device type only supports containers as it is for creating block devices with specific major and minor numbers which cannot be done with VMS.
Ok, both worked. I noticed that the path of the device in the container is /dev/sd?, as opposed to unix-block devices in containers that get the same path as the source property.
We do however set some metadata through QEMU so if you inspect /dev/disk/ you should find some symlinks that let you tie your LXD device name to the disk.