When migrating a guest from one host to another, the VNC connection is dropped. This is because the VNC connection is tied to the host. It would be good to allow the vnc console to work during a live migration, e.g. by broadcasting the vnc session or by sending it to a management host to which clients connect to.
This is outside the scope of virt-manager. If you want this behavior, the easiest thing to do would be to set up a VNC server inside the VM. Otherwise I think you would need to file a feature request with upstream qemu, then get the requisite support in libvirt (basically, a LARGE amount of work for something the user can manually configure). Closing as CANTFIX.