tests: monitor: json: Fix error message when returning json in json
    
    The qemu JSON monitor test allows to test also expected command
    arguments. As the error from the monitor simulator is returned as a
    simulated qemu error (in JSON) all other JSON contained in the error
    message needs to be escaped. This will happen if the monitor command
    under test receives a JSON array as an argument.
    
    This will improve the error message from:
    libvirt:  error : internal error: cannot parse json { "error":  { "desc":
    "Invalid value of argument 'keys' of command 'send-key': expected 'ble'
    got '[{"type":"number","data":43},{"type":"number","data":26},
    {"type":"number","data":46},{"type":"number","data":32}]'",
    "class": "UnexpectedCommand" } }: lexical error: invalid string in json text.
    
    To:
    libvirt: QEMU Driver error : internal error: unable to execute QEMU
    command 'send-key': Invalid value of argument 'keys' of command
    'send-key': expected 'ble' got '[{"type":"number","data":43},
    {"type":"number","data":26},{"type":"number","data":46},
    {"type":"number","data":32}]'
    
    This improvement will not have any effect on tests executing as
    expected, but it will help test development.