vmm: Deduplicate VM and vCPU state management code

Now that the machine-independent fields of struct vm and struct vcpu are
available in a header, we can move lots of duplicated code into
sys/dev/vmm/vmm_vm.c. This change does exactly that.

No functional change intended.

MFC after: 2 months
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D53585