Now that device selection has no sink/source-specific bits, we can have
generic device selection for this path. We do need to now track state
changes so we can look up the final device_id once the device is open,
though.
https://bugzilla.gnome.org/show_bug.cgi?id=740987
This is conceptually the right thing to do, and allows us to correctly
catch errors in device selection as well, which we could not do while
creating the ringbuffer.
https://bugzilla.gnome.org/show_bug.cgi?id=740987
