[kernel][vm] Life cycle for VmCowPages

Introduces a life cycle for VmCowPages that has an explicit transition
between Alive and Dead. This transition point is used to perform
cleanup, removing the need for the VmCowPagesContainer wrapper.

Change-Id: I8979c0d15435da6cb0e26349693b974c307506f7
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/975952
Commit-Queue: Adrian Danis <adanis@google.com>
Reviewed-by: Rasha Eqbal <rashaeqbal@google.com>
Reviewed-by: Sage Barreda <sagebarreda@google.com>