bhyve: Use MAP_GUARD when mapping guest memory ranges.

Instead of relying on PROT_NONE mappings with MAP_ANON, use MAP_GUARD
to reserve address space around guest memory ranges including the
guard ranges of address space around mappings.

Submitted by: Shawn Webb
Reviewed by: araujo
Approved by: re (rgrimes)
MFC after: 1 month
Sponsored by: HardendBSD and G2, Inc
Differential Revision: https://reviews.freebsd.org/D16822