bhyve: Move most early initialization into an MD routine

Prior to initializing PCI devices, main() calls a number of
initialization routines, many of which are amd64-specific. Move this
list of calls to bhyverun_machdep.c. Similarly, add an MD function to
handle late initialization.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40989