vmm.h: Add ABI assertions and mark implicit holes

The static assertions were added (with size and offsets from gdb) and verified
with a build prior to marking the holes explicitly.

This is in preparation for a subsequent revision, pending in phabricator, that
makes use of some of these unused bits without impacting the ABI.

Reviewed by: grehan
Differential Revision: https://reviews.freebsd.org/D24461