virt: propcan clean some old style construction.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: propcan change __slots__ listing to avoid waste a memory.

class X:
   __slots__ = ['a', 'b']

class Y(X):
   __slots__ = X.__slots__ + ['c']

Double declaring of slots causes a waste of memory
and aims to bad manners to slots.

Signed-off-by: Jiří Župka <jzupka@redhat.com>