bhyve: emulate CLFLUSH and CLFLUSHOPT.

Apparently CLFLUSH on mmio can cause VM exit, as reported in the PR.
I do not see that anything useful can be done except emulating page
faults on invalid addresses.

Due to the instruction encoding pecularity, also emulate SFENCE.

PR: 232081
Reported by: phk
Reviewed by: araujo, avg, jhb (all: previous version)
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D17482