Fix memory corruption in qjsvalueprivate

When converting to a memory managed string, we need to add
the PersistentValuePrivate to the chain of persistent objects.
Otherwise the string stored in the JSValue can get garbage
collected.

Change-Id: I159fd77a7ae6f29e4121969cac962bd12b0f9bc7
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>