Currently the format-specific compression option is ignored during creation of the restored qcow image (wihch is not that bad, because nowadays its zlib by default, anyway:

format-specific": {
        "type": "qcow2",
        "data": {
            "compat": "1.1",
            "compression-type": "zlib"

parse compression type and append it to qemu--img create command if set.

In addition the nbd server needs probably set options for compression, like:

qemu-nbd .. driver=compress,file.driver=qcow2,file.file.driver=file,file.file.filename=test.qcow2

otherwise restored blocks in the image end up uncompressed.