chain module: bugfixing and cosmetics

chain module: bugfixing and cosmetics

Bugs:

- one introduced ages ago - find_by_* functions should be tested vs. <0
  for failure
- one introduced by 579f15c7c456c - it accidentally blocked too much
  when testing for ',' in drive/partition option parsing
- one introduced by d4d713c9ccfe0 - sone machines / VMs seem to set
  @40:75 just 1, regardless of amount of fixed disks available

Signed-off-by: Michal Soltys <soltys@ziu.info>