Fixes #1373 - Fix disk stats computation in diskdata
Fixes #1373 - Fix disk stats computation in diskdata

When a disk was specified, _get_io_stats() was summing statistics
for all drives, not just the selected device name.

guiniol/py3status,tobes/py3status,Andrwe/py3status,valdur55/py3status,Andrwe/py3status,valdur55/py3status,guiniol/py3status,valdur55/py3status,ultrabug/py3status,tobes/py3status,ultrabug/py3status,ultrabug/py3status