[Feature] Enable to specify disks with path (symlink) to device file

Hi Dag,

this feature was originaly requested by Hitachi.

They had noted that:
" On each boot, device names (sdX) may be different
  (e.g. reboot after device hot-add or hot-remove).
  So, users would like to give e.g. SCSI ID (by-id) instead of
  sdX as dstat's argument. However, dstat command cannot accept SCSI ID."