bhyve: Move power management code to amd64/

This implements various x86-specific interfaces. No functional change
intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40735

(cherry picked from commit 75d1e855b0a2acfb60e5a3c81b018d8f73ebf450)