python: Reference parent Device object from Buffer
python: Reference parent Device object from Buffer

By having the Buffer objects reference their parent Device object,
without having the opposite, we can have a proper dependency graph, and
the objects will be freed in the correct order.

Signed-off-by: Paul Cercueil <a027184a55211cd23e3f3094f1fdc728df5e0500@crapouillou.net>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio