bhyve: add emulation for the qemu fwcfg selector port

The selector port is used to select the desired fwcfg item.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38332

(cherry picked from commit 151d8131a817e7a6a629e9bb7fde4d7a158e5211)