- Removed all own class attributes to avoid name clashes with delegated
class attributes.
- Implemented the previously missing Pool.usage_details property.
- Shadowed all class attributes as instance properties. This is required
as the parent classes enforce the class attributes upon the
CallbackPool & CallbackVolume classes, but they need to be delegated to
the class of the _cb_impl object. We also cannot implement them as class
attributes in CallbackVolume & CallbackPool as they need to work for
arbitrary backend drivers and two backend drivers must not interfere with
each other. Possible alternative: One could dynamically create classes.
Unfortunately this appears to be necessary due to
various Qubes OS `assert` checks and to get `__str__()` et al
from the super class. It also means that we have to implement
all methods of the super class (in the future as well).
