Translate mavlink python messages in json string

def mavlink_to_json(msg):
    '''Translate mavlink python messages in json string'''
    ret = '\"%s\": {' % msg._type
    for fieldname in msg._fieldnames:
        data = getattr(msg, fieldname)
        ret += '\"%s\" : \"%s\", ' % (fieldname, data)
    ret = ret[0:-2] + '}'
    return ret