virnetserverservice: Resolve Coverity ARRAY_VS_SINGLETON
    
    Coverity complained about the following:
    
    (3) Event ptr_arith:
       Performing pointer arithmetic on "cur_fd" in expression "cur_fd++".
    130             return virNetServerServiceNewFD(*cur_fd++,
    
    The complaint is that pointer arithmetic taking place instead of the
    expected auto increment of the variable...  Adding some well placed
    parentheses ensures our order of operation.