As this function is only used by runtimeVM::ContainerStats(), let's move
it to the runtime_vm.go file, making our life easier when doing upcoming
changes on runtimeVM::ContainerStats().

Signed-off-by: Fabiano Fidêncio <[email protected]>

CRI-O has been using `containers/libpod/pkg/cgroups` in order to get
metrics and, later on, convert it to CRI Stats. Although this approach
is fine (and desired) for the OCI runtime type. we can't rely on that
for the VM runtime type as the data sent by Kata Containers comes from
`containerd/cgroup`.

Signed-off-by: Fabiano Fidêncio <[email protected]>

WorkingSetBytes is the bit that needs to be set in order to provide
kubelet the pod's memory information.

Although it's calculated in a slightly different way for "oci" runtime
type, the logic is quite similar for the "vm" runtime type, with the
only difference being where the TotalInactiveFile information comes
from.

This is the last bit needed in order to have `kubectl top pod $pod`
working, as shown below:
```
[fidencio@localhost cri-o]$ kubectl get pods
NAME             READY   STATUS    RESTARTS   AGE
example-fedora   1/1     Running   0          130m

[fidencio@localhost cri-o]$ kubectl get pod example-fedora -o yaml | grep runtimeClassName
      {"apiVersion":"v1","kind":"Pod","metadata":{"annotations":{},"labels":{"app":"example-fedora-app"},"name":"example-fedora","namespace":"default"},"spec":{"containers":[{"args":["-m","http.server","8080"],"command":["python3"],"image":"fedora:33","name":"example-fedora","ports":[{"containerPort":8080}]}],"runtimeClassName":"kata"}}
        f:runtimeClassName: {}
  runtimeClassName: kata

[fidencio@localhost cri-o]$ kubectl top pod
NAME             CPU(cores)   MEMORY(bytes)
example-fedora   1m           9Mi
```

Signed-off-by: Fabiano Fidêncio <[email protected]>