vmm: Make vmm_dev.h more self-contained

vmm.h is required for VM_MAX_SUFFIXLEN. vmm_snapshot.h is required for
struct vm_snapshot_meta.

This is a prerequisite for including vmm_dev.h in the headers parsed by
libsysdecode.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D46485