virnetserverservice: Resolve Coverity ARRAY_VS_SINGLETON

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.