(0) Launch QEMU as following:
#----------------------------------------------------------------------
$ ~/build/build-qemu-block-stream-crash-test/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.filename=./a.qcow2  \
   -device virtio-blk,drive=node-A,id=virtio0 \
   -netdev user,id=net0,hostfwd=tcp::5555-:22 -device e1000,netdev=net0 \
   -monitor stdio -qmp unix:./qmp-sock,server,nowait
#----------------------------------------------------------------------

(1) Create an overlay:

(QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2                                                                                   
{
    "execute": "blockdev-snapshot-sync", 
    "arguments": {
        "node-name": "node-A", 
        "snapshot-file": "b.qcow2", 
        "format": "qcow2", 
        "snapshot-node-name": "node-B"
    }
}
{
    "return": {}
}
#----------------------------------------------------------------------


(2) Create a target image, and add it via `blockdev-add`:
#----------------------------------------------------------------------

$ qemu-img create -f qcow2 backup1.qcow2 1G                                                                                                                        
Formatting 'backup1.qcow2', fmt=qcow2 size=1073741824 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16


(QEMU) blockdev-add driver=qcow2 node-name=node-backup1 file={"driver":"file","filename":"/export/backup1.qcow2"}                                                                             
{
    "execute": "blockdev-add", 
    "arguments": {
        "node-name": "node-backup1", 
        "driver": "qcow2", 
        "file": {
            "driver": "file", 
            "filename": "/export/backup1.qcow2"
        }
    }
}
{
    "return": {}
}
(QEMU) 
#----------------------------------------------------------------------

(3) Perform `blockdev-backup`, to the target by copying the entire chain [A] <-- [B]
#----------------------------------------------------------------------

(QEMU) blockdev-backup device=node-B target=node-backup1 sync=full job-id=job0
{
    "execute": "blockdev-backup", 
    "arguments": {
        "device": "node-B", 
        "job-id": "job0", 
        "target": "node-backup1", 
        "sync": "full"
    }
}
{
    "return": {}
}
(QEMU) 
#----------------------------------------------------------------------

(4) Try to examine the disk image, and there's no operating system detected:
#----------------------------------------------------------------------
$ du -sh examine-backup1.qcow2 
22M     examine-backup1.qcow2

$ export LIBGUESTFS_BACKEND=direct
$ guestfish --ro -i -a examine-backup1.qcow2                                                                                                                       
guestfish: no operating system was found on this disk

If using guestfish '-i' option, remove this option and instead
use the commands 'run' followed by 'list-filesystems'.
You can then mount filesystems you want by hand using the
'mount' or 'mount-ro' command.

If using guestmount '-i', remove this option and choose the
filesystem(s) you want to see by manually adding '-m' option(s).
Use 'virt-filesystems' to see what filesystems are available.

If using other virt tools, this disk image won't work
with these tools.  Use the guestfish equivalent commands
(see the virt tool manual page).

#----------------------------------------------------------------------

(5) However, when you examine the overlay file, b.qcow2, the OS *is* still there.
#----------------------------------------------------------------------
$ guestfish --ro -i -a b.qcow2 

Welcome to guestfish, the guest filesystem shell for
editing virtual machine filesystems and disk images.

Type: 'help' for help on commands
      'man' to read the manual
      'quit' to quit the shell

Operating system: 0.3.5
/dev/sda1 mounted on /

><fs> 
#----------------------------------------------------------------------