bhyve: add helper for adding fwcfg files

Fwcfg items without a fixed index are reported by the file_dir. They
have an index of 0x20 and above. This helper simplifies the addition of
such fwcfg items. It selects a new free index, assigns it to the fwcfg
items and creates an proper entry in the file_dir.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38336

(cherry picked from commit 6f9ebb3d0fed2b0ae604dd6daf17f1fe1d8df216)