ovs: wait that link disappears before continuing with deactivation
When we deactivate a virtual device, we usually schedule the deletion of the link in an idle handler. That action will be executed at a later time when the device is already in the disconnected state.
Similarly, for ovs interfaces we send the deletion command to the ovsdb and then proceed to the disconnected state.
However, in the first case there is the guarantee that the link will be deleted at some point, while for ovs interfaces it may happen that ovs decides to reuse the same link if there is an addition queued. Since reusing the same link confuses NM, let's implement deactivate_async() for ovs-interfaces and wait that the link actually goes away before proceeding.