Allow node selection

We have a separate node class with rke2 set up for metal nodes as AWS doesn't support nested virt and pure qemu emulation is way too slow for production environments. The rke2 metal worker nodes are tainted to restrict them to kubevirt VMs only. This requires setting up the nodeSelector and tolerations under the 'workloads' section of the KubvirtVirt resource.

# Example
---
apiVersion: kubevirt.io/v1
kind: KubeVirt
metadata:
  name: kubevirt
  namespace: kubevirt-system
spec:
  workloads:
    nodePlacement:
      nodeSelector:
        node.kubernetes.io/pool: "metal"
      tolerations:
        - key: "metal"
          operator: "Equal"
          value: "True"
          effect: "NoSchedule"
...

We need a way to enable and customize this through the helm templates.