Change the lower bound for guest vmspace allocation to 0 instead of
using the VM_MIN_ADDRESS constant.

HardenedBSD redefines VM_MIN_ADDRESS to be 64K, which results in
bhyve VM startup failing. Guest memory is always assumed to start
at 0 so use the absolute value instead.

Reported by:	Shawn Webb, lattera at gmail com
Reviewed by:	neel, grehan
Obtained from:	Oliver Pinter via HardenedBSD
https://github.com/HardenedBSD/hardenedBSD/commit/23bd719ce1e3a8cc42fc8317b1c7c6d9e74dcba0
MFC after:	1 week
Workaround help2man defficiency that prevents mandoc from rendering