scripts/shutdown-vmhost: Skip Longhorn nodes

We **DO NOT** want to shut down the Longhorn Kubernetes nodes!  Doing so
would pretty much nuke everything running in the cluster.  The shutdown
script will need to migrate them online; fortunately, since they don't
run anything except Longhorn, they should be able to migrate fine.

scripts/shutdown-vmhost.sh

@@ -63,7 +63,7 @@ virsh uri || exit
 
 set --
 nodes=$(
-    kubectl get node -o name -l '!node-role.kubernetes.io/control-plane' \
+    kubectl get node -o name -l '!node-role.kubernetes.io/control-plane,!node-role.kubernetes.io/longhorn' \
     | sed s@node/@@
 )
 for node in ${nodes}; do