Use boost::optional to simplify VxCreator

• instead of keeping a seperate bool to flag the presence of a 'dummy' trivial value, use boost::optional instead, which avoids having to create a 'dummy', and which avoids the corresponding bookkeeping. This fixes the longstanding warning about hiding virtual functions, as the instance which caused the warning isn't instantiated anymore...
• replace a deprecated std::auto_ptr with std::unique_ptr