bhyve: Push option parsing down into bhyverun_machdep.c

After a couple of attempts I think this is the cleanest approach despite
the expense of some code duplication. Quite a few of the single-letter
bhyve options are x86-specific.

I think that going forward we should strongly discourage the addition of
new options and instead configure guests using the more general
configuration file syntax.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41753