fix segfault for diskiograph

fix segfault for diskiograph

The crux is to split args between those for scan_graph() on one hand and
prepare_diskio_stat() on the other hand. To make the code working with
minimal changes, move the yet optional devicename to the end.

General note on graphs:
- for all graphs there exist the optional arguments for scan_graph()
  which come first and are optional (intended duplicate)
- all other args are object specific, and may be optional as well