vmm: Add support for guest NUMA emulation

This change adds the necessary kernelspace bits required for
supporting NUMA domains in bhyve VMs.

The layout of system memory segments and how they're created has
been reworked. Each guest NUMA domain will now have its own memory
segment. Furthermore, this change allows users to tweak the domain's
backing vm_object domainset(9) policy.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44565