The QEMU commit (along with other supporting commits) added '-blockdev'
implementation that is available from QEMU 2.9 release onwards.

    commit 42e5f39378c6e7a0ada563779bbb6f470f7c03ff
    Author: Markus Armbruster <armbru@redhat.com>
    Date:   Tue Feb 28 22:27:07 2017 +0100
    
        block: Initial implementation of -blockdev
    [...]

Examples
========

[I included the '-qmp' QMP server, but that optional, I added it here
for my own convenience.]

Example-1: The traditiona dotted-key syntax:
-----------------------------------------------------------------------
$ ./x86_64-softmmu/qemu-system-x86_64 -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 \
    -monitor stdio -qmp unix:./qmp-sock,server,nowait
-----------------------------------------------------------------------

Example-2: The equivalent JSON syntax for '-blockdev':
-----------------------------------------------------------------------
$ ./x86_64-softmmu/qemu-system-x86_64 -display none -nodefconfig \
    -nodefaults -m 512 \
    -blockdev '{"node-name": "node-A", "driver": "qcow2", "file": {"driver": "file", "filename": "a.qcow2"} }'
    -device virtio-blk,drive=node-A,id=virtio0 \
    -monitor stdio -qmp unix:./qmp-sock,server,nowait
-----------------------------------------------------------------------

* * *

Notes
=====

Since QEMU 2.9, command-line '-blockdev' is a flattenned mapping of
QMP `blockdev-add`:

Therefore, QMP `blockdev-add`:
-----------------------------------------------------------------------
{
    "execute":"blockdev-add",
    "arguments":{
        "driver":"qcow2",
        "node-name":"node1",
        "file":{
            "driver":"file",
            "filename":"./disk1.qcow2"
        }
    }
}
-----------------------------------------------------------------------

Becomes the following command-line '-blockdev' equivalent:
-----------------------------------------------------------------------
    $ qemu-system-x86 [...]\ 
        -blockdev driver=qcow2,node-name=node1,
        \ file.driver=file,file.filename=./disk1.qcow2
        [...]
-----------------------------------------------------------------------


Additional info
===============

To create an overlay chain:

[A] <-- [B] <-- [C] <-- [D]

Using the QMP `blockdev-snapshot-sync`, using 'qmp-shell', use the
following (so that 'node-name' parameters are included):

-----------------------------------------------------------------------
$ ./qmp-shell -v -p /tmp/qmp-sock
(QEMU)
(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": {}
}
-----------------------------------------------------------------------

And so on for further overlays, [C], and [D]:

(QEMU) blockdev-snapshot-sync node-name=node-B snapshot-file=c.qcow2 snapshot-node-name=node-C format=qcow2                                                                                   
(QEMU) blockdev-snapshot-sync node-name=node-C snapshot-file=d.qcow2 snapshot-node-name=node-D format=qcow2