bhyve: Split vmexit handling into a separate file

Put it in amd64, since most of it is MD and won't be used on arm64. Add
a bit of glue to bhyverun.h to make CPU startup and shutdown work
without having to export more global variables. AP startup will be
reworked further in a future revision.

This makes bhyverun.c much more machine-independent.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40556