config: make sure we create the config file properly

I guess this code was tested with an existing config file only, because
I have the following:

DEBUG: tuhi.config: E0:61:C6:BF:14:4E: adding new config, uuid 8bbc6be4347a
Traceback (most recent call last):
  File "./tuhi.py", line 153, in _on_uuid_updated
    self.config.new_device(bluez_device.address, wacom_device.uuid)
  File "tuhi/tuhi/config.py", line 89, in new_device
    config['Device']['Address'] = address
  File "/usr/lib64/python3.6/configparser.py", line 959, in __getitem__
    raise KeyError(key)
KeyError: 'Device'

tuhi/config.py

@@ -86,8 +86,11 @@ class TuhiConfig(GObject.Object):
         config.optionxform = str
         config.read(path)
 
-        config['Device']['Address'] = address
-        config['Device']['UUID'] = uuid
+        config['Device'] = {
+            'Address': address,
+            'UUID': uuid,
+        }
+
         with open(path, 'w') as configfile:
             config.write(configfile)