vmm: Move duplicated stats code into a generic file

There is a small difference between the arm64 and amd64 implementations:
the latter makes use of a "scope" to exclude AMD-specific stats on Intel
systems and vice-versa. Replace this with a more generic predicate
callback which can be used for the same purpose.

No functional change intended.

Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D46430