Bhyve's fwctl only passes the number of vcpus to the guest. That could be done easily by QEMU's fwcfg. Additionally, QEMU's fwcfg has much more features. Sadly, bhyve's fwctl uses the same ports as QEMU's fwcfg. For that reason, it's not possible to use them both simultaneously.
Add `-l fwcfg,qemu` to your bhyve cmdline to enable QEMU's fwcfg.
Requires a firmware patch:
{F27158932}