RFE: QMP: report error reason in BLOCK_IO_ERROR message

when a disk drive is configured with the error policy enospc for write errors, the monitoring client needs a way to distinguish
betwwen generic I/O error and the I/O error for space exausted.

The JSON QMP protocol lacks this information: the BLOCK_IO_ERROR message does not provide a reason or code for the error verified, so the monitoring client cannot distinguish the source of the errors.

verified against git 105a060188dc6fdd4551571a966514d1a5f6815a