This patch splits out the part of virNetDevTapCreateInBridgePort()
that would need to be re-done if an existing tap device had to be
re-attached to a bridge, and puts it into a separate function. This
can be used both when an existing domain interface config is updated
to change its connection, and also to re-attach to the "same" bridge
when a network has been stopped and restarted. So far it is used for
nothing.