Qt online installer incorrectly determines free disk space when the install path contains a symbolic link

If we install Qt in '/opt/Qt'  where  '/opt' is a symbolic link to another partition than '/'  ( e.g. '/home/opt/Qt'  ) then the installer check for free disk space in the '/' partition instead of the corresponding partition.

Fix: The install path must be expanded to an absolute path to get ride of partition folk.