(1) Launch the source and destination QEMU as below:

SRC:
#----------------------------------------------------------------------
$ ~/build/qemu-upstream/x86_64-softmmu/qemu-system-x86_64 -M q35 -display none -nodefconfig -nodefaults -m 512 -blockdev node-name=node-A,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./a.qcow2 -device virtio-blk,drive=node-A,id=virtio0 -netdev user,id=net0,hostfwd=tcp::5555-:22 -device e1000,netdev=net0 -S -monitor stdio -qmp unix:./qmp-sock,server,nowait
#----------------------------------------------------------------------

DEST:
#----------------------------------------------------------------------
$ qemu-img create -f qcow2 dest-disk.qcow2 39M
#----------------------------------------------------------------------
$ ~/build/qemu-upstream/x86_64-softmmu/qemu-system-x86_64 -M q35 -display none -nodefconfig -nodefaults -m 512 -blockdev node-name=node-DestDisk,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./dest-disk.qcow2 -device virtio-blk,drive=node-DestDisk,id=virtio0 -netdev user,id=net0,hostfwd=tcp::7777-:22 -device e1000,netdev=net0 -S -monitor stdio -qmp unix:./qmp-sock2,server,nowait
#----------------------------------------------------------------------


(2) On DEST QEMU (`qmp-shell`):
#----------------------------------------------------------------------
$ qmp-shell -v -p /export/qmp-sock2

(QEMU) nbd-server-start addr={"type":"inet","data":{"host":"::","port":"49153"}}
{
    "execute": "nbd-server-start", 
    "arguments": {
        "addr": {
            "data": {
                "host": "::", 
                "port": "49153"
            }, 
            "type": "inet"
        }
    }
}
{
    "return": {}
}
(QEMU) 
#----------------------------------------------------------------------

(QEMU) nbd-server-add device=node-DestDisk writable=true
{
    "execute": "nbd-server-add", 
    "arguments": {
        "device": "node-DestDisk", 
        "writable": true
    }
}
{
    "error": {
        "class": "GenericError", 
        "desc": "Conflicts with use by /machine/peripheral/virtio0/virtio-backend as 'root', which does not allow 'write' on node-DestDisk"
    }
}
#----------------------------------------------------------------------