bhyve: add cmdline option for user defined fw_cfg items

Some guest allow to configure themself by fw_cfg. E.g. Fedora CoreOs can
be provisioned by adding a JSON file as fw_cfg item.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38338