Refactor QEMU socket connection handling and start vmop agent.

webpages/vm-operator/upgrading.md

@@ -26,6 +26,13 @@ layout: vm-operator
    still accepted for backward compatibility until the next major version,
    but should be updated.
 
+ * The standard [template](./runner.html#stand-alone-configuration) used
+   to generate the QEMU command has been updated. Unless you have enabled
+   automatic updates of the template in the VM definition, you have to
+   update the template manually. If you're using your own template, you
+   have to add a virtual serial port (see the git history of the standard
+   template for the required addition).
+
 ## To version 3.4.0
 
 Starting with this version, the VM-Operator no longer uses a stateful set