Fix virt BDM __setattr__ and __getattr__

Previously we would only allow these to work on fields that are found in
the _proxy_as_attr collection of the object and we would unconditionally
raise an AttributeError otherwise.

This behaviour is too restrictive as it prevents us from using mock on
these objects properly, Instead we now delegate back to the parent
class for a more natural behaviour.

Change-Id: I5c221eef8f05c66ac2e2fb4abf645748407462c0
Merge "Fix virt BDM __setattr__ and __getattr__"