Add libvirt values.yaml

Change-Id: I28253a8cb609e59f4368502e8b81116d54532950
diff --git a/prereqs/openstack-helm.md b/prereqs/openstack-helm.md
index d60f9a8..7627a45 100644
--- a/prereqs/openstack-helm.md
+++ b/prereqs/openstack-helm.md
@@ -61,6 +61,15 @@
 ```
 
 ```bash
+cat <<EOF > /tmp/libvirt-cord.yaml
+---
+network:
+  backend: []
+EOF
+export OSH_EXTRA_HELM_ARGS_LIBVIRT="-f /tmp/libvirt-cord.yaml"
+```
+
+```bash
 cat <<EOF > /tmp/nova-cord.yaml
 ---
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