storage_driver: Resolve Coverity REVERSE_INULL
    
    There were two occurrances of attempting to initialize actualType by
    calling virStorageSourceGetActualType(src) prior to a check if (!src)
    resulting in Coverity complaining about the possible NULL dereference
    in virStorageSourceGetActualType() of src.
    
    Resolve by moving the actualType setting until after checking !src