Code review comment for lp://staging/~raof/qtmir/multi-bufferstream-support

Oh, grr. In updating this against latest trunk I discover that we need to make some architectural decisions before qtmir has the information required to do this correctly. See also: https://docs.google.com/document/d/1Se4kbUHZNbgB9e3lrr3EQnVnQgSgFThyiBJ0MDgkZ0c