bhyve: Make boot ROM handling more consistent
- On amd64, deprecate lpc.bootrom and lpc.bootvars. Use top-level config variables instead.
- Introduce a generic predicate which can be used to determine whether the guest has a boot ROM.
Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D46282