docs/qmp: Fix documentation of BLOCK_JOB_READY to match code

Data:

- "device": device name (json-string)

- "type":     Job type (json-string; "stream" for image streaming

                                     "commit" for block commit)

- "device":   Device name (json-string)

- "len":      Maximum progress value (json-int)

- "offset":   Current progress value (json-int)

              On success this is equal to len.

              On failure this is less than len.

- "speed":    Rate limit, bytes per second (json-int)

Example:

{ "event": "BLOCK_JOB_READY",

    "data": { "device": "ide0-hd1" },

    "data": { "device": "drive0", "type": "mirror", "speed": 0,

              "len": 2097152, "offset": 2097152 }

    "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }

Note: The "ready to complete" status is always reset by a BLOCK_JOB_ERROR