bhyve: add cmdline option to enable qemu's fwcfg

Let the user decide if he wants to use bhyve's fwctl or qemu's fwcfg. He
can set the interface by adding a fwcfg option to bootrom:

-l bootrom,<path/to/rom>,fwcfg=bhyve
-l bootrom,<path/to/rom>,fwcfg=qemu

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38337

(cherry picked from commit d85147f3d6811c04e250bfb4597783d8844de315)