bhyve: dynamically register FwCtl ports

Qemu's FwCfg uses the same ports as Bhyve's FwCtl. Static allocated
ports wouldn't allow to switch between Qemu's FwCfg and Bhyve's
FwCtl.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33496