bhyve: add common fwcfg items

[corvink]

Summary

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.

[markj]

usr.sbin/bhyve/qemu_fwcfg.c

193

200

Why a negative value?

208

This line is too long, missing parens around the return value.

[corvink]

• fix style

[markj]

usr.sbin/bhyve/qemu_fwcfg.c

203

This style of commenting is useless IMHO. It would be much more useful to, for example, describe the information that is encoded in this directory structure.