protocol:
{
  "driver": "file",
  "filename": "/var/lib/libvirt/images/i.qcow2",
  "size": 42
}

format:
{
  "driver": "qcow2",
  "file": "0123456789ABCDEF0123456789ABCDE",
  "size": 1337,
  "backing-file": "json:{\"driver\":\"nbd\",\"server\":{\"type\":\"inet\",\"host\":\"example.com\",\"port\":\"1234\"}}",
  "backing-fmt": "raw"
}