bhyve: add cmdline option for user defined fw_cfg items

[corvink]

Summary

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.

[corvink]

• rebase

[corvink]

@markj @jhb Any complains about this? I'd like to get this merged for 14.0.

[corvink]

• fix minor style issues

[corvink]

HEADS UP. I'm going to commit this tomorrow if noone complains.

[markj]

usr.sbin/bhyve/qemu_fwcfg.c

407

584

Why not just use fstat()?

595

This is missing handling of errors and short reads.

[corvink]

• fix style issues
• make use of fstat
• handle short reads

[corvink]

@jhb @markj Any additional feedback?

[jhb]

A few suggestions, but looks ok to me otherwise.

usr.sbin/bhyve/bhyverun.c

1294

EX_USAGE?

usr.sbin/bhyve/qemu_fwcfg.c

604–606

[markj]

usr.sbin/bhyve/qemu_fwcfg.c

567

583

This can be warn() instead of warnx().

585

close() would generally clobber errno. So we either need to save/restore it, or just return -1.

601

Same comment about errno. Also a short read is not necessarily an error.