Some fixes to abstractwizard

* Test the batch_selection_dialog's type using issubclass. isinstance is just
  for objects and will not work
* sellable_selected was being called twice (even worse, the duplicated call was
  not passing batch so if a user put a batch_number on the barcode it would
  still ask for the batch)

Fixes: http://bugs.async.com.br/show_bug.cgi?id=5590
Change-Id: I7bb646ff18d64c8e31416d378da2f94ce9ad5d50