guest: unify pod model for V1, virtual pod, and V2 shim support

[shreyanshjain7174]

The GCS guest runtime (internal/guest/runtime/hcsv2/uvm.go) tracks virtual pods separately from V1 sandbox containers — a dedicated VirtualPod type, seven exported methods, a parent cgroup manager, and a reverse-lookup map. V1 sandboxes have no pod-level tracking at all. Adding V2 shim support would need a third path.

This collapses all three into one: a private uvmPod type and a single pods map on Host. Every sandbox — V1, virtual pod, or V2 shim — goes through createPodInUVM, which allocates a cgroup under /pods/{sandboxID}. Workload containers nest at /pods/{sandboxID}/{containerID}. Container-to-pod membership is tracked via addContainerToPod. Cleanup in RemoveContainer is a single code path: remove the container from the pod, and when the sandbox container itself is removed, delete the pod's cgroup.

Cgroup hierarchy changes from:

/containers/{id}                         (V1 sandbox)
/containers/virtual-pods/{virtualPodID}  (virtual pod)

to:

/pods/{sandboxID}                        (all pod types)
/pods/{sandboxID}/{containerID}          (workload containers)

Standalone (non-CRI) containers keep their own cgroup at /pods/{id} with no pod entry — same isolation as before, just under the new prefix.

Network namespace teardown for virtual pod sandboxes is preserved: RemoveContainer skips RemoveNetworkNamespace for virtual pod sandbox containers since the host-driven path (TearDownNetworking → RemoveNetNS → removeNIC) handles adapter removal first.

cmd/gcs/main.go replaces the /containers/virtual-pods parent cgroup with /pods and drops the InitializeVirtualPodSupport call.

Tested E2E with both shims:

	V1 shim (io.containerd.runhcs.v1)	V2 shim (io.containerd.lcow.v2)
OCIBundlePath	/run/gcs/c/<podId>	/run/gcs/pods/<podId>/<podId>
Pod cgroup	/sys/fs/cgroup/memory/pods/<podId>	/sys/fs/cgroup/memory/pods/<podId>
/containers/virtual-pods/	absent	absent