klog prints an internal error when trying to log the podActions struct.
> I0505 14:12:12.827065 190662 kuberuntime_manager.go:1014] "computePodActions got for pod" podActions="<internal error: json: unsupported type: map[container.ContainerID]kuberuntime.containerToKillInfo>" pod="kube-system/coredns-8f5847b64-mzw46"
Implement the stringer interface on the struct to avoid the json error.
Change-Id: I22444524a78a0ecec9490b9240def371a4129434