Inconsistent supported_models between class and instance objects

[rytilahti]

In [1]: from miio import AirPurifierMiot                                                                                                                                                                                                  

In [2]: AirPurifierMiot.supported_models                                                                                                                                                                                                  
Out[2]: dict_keys(['zhimi.airpurifier.ma4', 'zhimi.airpurifier.mb3', 'zhimi.airpurifier.va1', 'zhimi.airpurifier.vb2', 'zhimi.airpurifier.mb4', 'zhimi.airp.mb4a', 'zhimi.airp.mb5', 'zhimi.airp.va2', 'zhimi.airp.vb4', 'zhimi.airp.rmb1', 'zhimi.airpurifier.za1'])

In [3]: dev = AirPurifierMiot("127.0.0.1", 32*" ")                                                                                                                                                                                        

In [4]: dev.supported_models                                                                                                                                                                                                              
Out[4]: []

Originally posted by @rytilahti in #1447 (comment)