The way I see block filter currently implemented is as a special block device with `is_filter` set to true.
Is this a correct characterization of the current incarnation? If so, I was wondering if it is possible to "insert" a block filter layer on top of an existing block device once QEMU is executing (via QMP commands)? It seems possible to add block filter managed block devices, but I don't think I see a way of adding a block filter to an existing block device. -- Wolf
