bhyve: add helper to add fwcfg items

This helper makes it easier to add multiple fwcfg items. You can pass an
index and some data to the helper. The helper adds these information to
the fwcfg emulation so that the guest reads the given data on the
specified index.

Reviewed by: <If someone else reviewed your modification.>
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38334

(cherry picked from commit 3ef46195ac37111ab247cfabed735d3ee5e7f5b5)