bhyve: add support for MTRR

Some guests or driver might depend on MTRR to work properly. E.g. the
nvidia gpu driver won't work without MTRR.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33333