Launch QEMU:

$> qemu-system-x86_64 -display none -M q35 -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 \
    -monitor stdio -qmp unix:/tmp/qmp-sock,server,nowait


$> ./qmp-shell -v -p /tmp/qmp-sock 
Welcome to the QMP low-level shell!
Connected to QEMU 3.1.0
(QEMU) query-version
{
    "execute": "query-version", 
    "arguments": {}
}
{
    "return": {
        "qemu": {
            "micro": 0, 
            "major": 3, 
            "minor": 1
        }, 
        "package": "qemu-3.1.0-4.fc28"
    }
}

(QEMU) blockdev-create job-id=job0 options={"driver":"qcow2","size":16384,"file":{"driver":"file","filename":"/export/target.qcow2"}}
{
    "execute": "blockdev-create", 
    "arguments": {
        "job-id": "job0", 
        "options": {
            "driver": "qcow2", 
            "file": {
                "driver": "file", 
                "filename": "/export/target.qcow2"
            }, 
            "size": 16384
        }
    }
}
{
    "return": {}
}
(QEMU) 
{u'timestamp': {u'seconds': 1549372257, u'microseconds': 740408}, u'data': {u'status': u'created', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}
{u'timestamp': {u'seconds': 1549372257, u'microseconds': 740439}, u'data': {u'status': u'running', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}
{u'timestamp': {u'seconds': 1549372257, u'microseconds': 740539}, u'data': {u'status': u'aborting', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}
{u'timestamp': {u'seconds': 1549372257, u'microseconds': 740557}, u'data': {u'status': u'concluded', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}

(QEMU) job-dismiss id=job0
{
    "execute": "job-dismiss", 
    "arguments": {
        "id": "job0"
    }
}
{
    "return": {}
}

(QEMU) 
{u'timestamp': {u'seconds': 1549372261, u'microseconds': 595150}, u'data': {u'status': u'null', u'id': u'job0'}, u'event': u'JOB_STATUS_CHANGE'}

(QEMU) blockdev-add driver=qcow2 node-name=node-Target file={"driver":"file","filename":"/export/target.qcow2"} backing={"driver":"qcow2","file":{"driver":"file","filename":"/export/a.qcow2"}}
{
    "execute": "blockdev-add", 
    "arguments": {
        "backing": {
            "driver": "qcow2", 
            "file": {
                "driver": "file", 
                "filename": "/export/a.qcow2"
            }
        }, 
        "node-name": "node-Target", 
        "driver": "qcow2", 
        "file": {
            "driver": "file", 
            "filename": "/export/target.qcow2"
        }
    }
}
{
    "error": {
        "class": "GenericError", 
        "desc": "Could not open '/export/target.qcow2': No such file or directory"
    }
}
(QEMU)