Attached disk blockpull

[Yoonho Park]

I am trying to perform a blockpull on an attached disk, but I cannot do this using "virsh attach-disk" specifying a new, empty overlay file created with "qemu-img create". The VM does not pick up the backing_file path from the qcow2 overlay file, and there seems to be no way to specify the backing path through the attach-disk command itself. Instead the backing path has to be specified in the xml file used to create the VM, which precludes attaching a new disk to a running VM. Is this a bug? Is it possible to attach a disk to a running VM, and specify its backing path, using qemu directly? This is with qemu 4.2.0 and libvirt 6.0.0.