bhyve: Push option parsing down into bhyverun_machdep.c

[markj]

Summary

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.

[jhb]

I agree that we should be using '-o' for new things going forward.