Add virtual qualifier to derived Operation class destructors

When a base class destructor is virtual, derived class destructors are
also virtual [1] even if they don't have the virtual qualifier.

As the Operation destructor is virtual, derived Operation* classes
destructors are virtual too.  Add virtual qualifier just to reflect what
the C++ language mandates the compiler implement.

[1] Derived class with non-virtual destructor
    http://stackoverflow.com/questions/7403883/derived-class-with-non-virtual-destructor