D-Bus clients can call CreateInterface() once and use the resulting
Interface object to connect multiple times to different networks.
However, if the network interface gets added to a bridge, clients
currently have to remove the Interface object and create a new one.
Improve this by supporting the change of the BridgeIfname property of
an existing Interface object.
Signed-off-by: Beniamino Galvani <bgalvani@redhat.com>