bhyve: Move vcpu initialization into a MD source file

• Make handling of x86 config options, like x86.x2apic, conditional to amd64.
• Move fbsdrun_set_capabilities() and spinup_vcpu() to a new file, bhyverun_machdep.c. The moved code is all highly x86 specific.

I'm not sure how best to handle the namespace. I'm using "bhyve_" for
MD functions called from MI code. We also have "fbsdrun_" for some MI
routines that are typically called from MD code. The file name is
prefixed by "bhyverun_".

Reviewed by: corvink
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40987

(cherry picked from commit e20b74da223d675321618fe6d67858084d152c9e)