Add some warnings around virsh-cleanup

I actually had some other VMs; luckily in my case they're all
easy to recreate, but others may not be in the same situation.

scripts/maintenance/virsh-cleanup

@@ -1,5 +1,13 @@
 #!/bin/sh
 
+printf 'Warning: This will destroy effectively all libvirt resources\nContinue [yN]? '
+read CONTINUE
+if test "${CONTINUE}" != y -a "${CONTINUE}" != Y
+then
+	  echo 'Aborted' >&2
+	  exit 1
+fi
+
 CONNECT="${CONNECT:=qemu:///system}"
 POOL="${POOL:=default}"