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: NJiří Župka <jzupka@redhat.com>

Conflicts:
	virttest/utils_net.py
	virttest/virsh.py

Conflicts:
	virttest/libvirt_xml/devices/address.py
	virttest/libvirt_xml/network_xml.py