Make error reporting for QEMU JSON mode more friendly

Make error reporting for QEMU JSON mode more friendly

Current error reporting for JSON mode returns the full JSON
command string and full JSON error string. This is not very
user friendly, so this change makes the error report only
contain the basic command name, and friendly error message
description string. The full JSON data is logged instead.

* src/qemu/qemu_monitor_json.c: Always return the 'desc' field from
  the JSON error message to users.