bhyve: add basic qemu fwcfg implementation

qemu's fwcfg and bhyve's fwctl are both used to configure ovmf. qemu's
fwcfg is much more powerfull than bhyve's fwctl. For that reason, add
support for qemu's fwcfg.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38331