QtIFW

Improve volume information.

- Remove some superfluous functions.
- Store and compare the drive identifier only.
- Rename some functions to better reflect there purpose.

OSX/Linux:
- Store mounted device instead of volume label.

Windows:
- Remove dead code.
- Take mounted volumes into account.
- Store a unique name as drive identifier.
- Use the first parameter from GetDiskFreeSpaceEx, as that one
  considers possible quota values per-user as well. If per-user
  quotas are being used, this value may be less than the total
  number of free bytes on a disk retrieved with the third parameter.
- Make sure we properly check the path. In case of mounted volumes,
  a "\" might be the last character on the volume info path. If we
  just check for starts with, we might not notice that
  e.g. c:\test == c:\test\.

Change-Id: Ib04e7ccf3a7a3d3d156753fcd18785201fe87c25
Reviewed-by: Tim Jenssen <tim.jenssen@nokia.com>