vmm: Move the guest vmspace into the generic vm_mem structure

This further consolidates handling of guest memory into MI code in
sys/dev/vmm.

No functional change intended.

Reviewed by: corvink
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D53012

(cherry picked from commit 059b0b7046639121f3dca48f5de051e019f9d57c)