bhyve: add emulation for qemu's fwcfg data port

The data port returns the data of the fwcfg item.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38333

(cherry picked from commit b11081dca76b8e283495632b86b41539ee013857)