The p:ajax is OK, if it's not calling the bean, try to update your PF to 4.0
If you want to get the picklist target on your commandbutton, just use : picklist.getTarget();
If you want to do that in your onTransfer event, do the following:
private DualListModel<yourEntity> allVisas = new DualListModel<yourEntity>();
public void onTransfer(){
List<yourEntity> target = allVisas.getTarget();
}