When an object is retrieved from the datastore by JDO, only the called fetch group is retrieved. Unindexed properties including Text and Blob are not retrieved by default. This means that serialized field values (as your Round objects) will not be retrieved by default. This is called lazy loading.*
To get only certain fieds you can use a query like this:
query = pm.newQuery("SELECT status, players FROM " +
Game.class.getName() + " WHERE xy == :xy);
Remember that in this case the fields you retrieve must be indexed.
Hope this helps.
*REF: Google App Engine Java and GWT Application Development