Launch QEMU this way:

$ ./x86_64-softmmu/qemu-system-x86_64 -display none -nodefconfig \
    -M q35 -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:/tmp/qmp-sock,server,nowait


Interact with the QEMU via `qmp-shell`:

$ ./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
(QEMU) block-commit device=node-B base=a.qcow2 top=b.qcow2 job-id=job0

    # Here's The Glitch: you won't see the BLOCK_JOB_{COMPLETED,READY}                                                                                                                        
    # events *until* you hit the enter key right after `block-commit`                                                                                                                         
    # completes.

(QEMU) query-block-jobs
(QEMU) block-job-complete device=job0