bhyve: Move MSR emulation into amd64/

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40733

(cherry picked from commit 548b11228f7a83785dcf6c4c8b0bbfed0284d794)