bhyve: add common fwcfg items

Qemu defines some common fwcfg items. We don't need to support all of
them. Only a subset needs to be present for fwcfg to work properly.

• signature The signature is used by the guest to check if qemu's fwcfg is available or not.
• id The id is used by the guest to check which features are supported by the fwcfg implementation of the hypervisor.
• file_dir The file dir reports all fwcfg items which don't have a fixed index. These are mostly user defined fwcfg items.

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/D38335