vmm: Expose more registers to VM_GET_REGISTER

In a follow-up revision the gdb stub will support sending an XML target
description to gdb, which lets us send additional registers, including
the ones added in this patch.

Reviewed by: jhb
MFC after: 1 month
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D43665

(cherry picked from commit f493ea650e6137ba657dfa0627da1e8bb4a985e9)