QStorageInfo: get the label properly if the mounted device is a symlink

This happens for me with LVM:

 $ ls -l /dev/mapper/system-root /dev/system/root /dev/disk/by-label/*
 lrwxrwxrwx 1 root root 10 Nov 17 22:45 /dev/disk/by-label/system -> ../../dm-1
 lrwxrwxrwx 1 root root  7 Nov 17 22:45 /dev/mapper/system-root -> ../dm-1
 lrwxrwxrwx 1 root root  7 Nov 17 22:45 /dev/system/root -> ../dm-1

The mounted device according to /etc/mtab (/proc/self/mounts) is
/dev/mapper/system-root.

Change-Id: I3e15a26e0e424169ac2bffff1417b96779d84246
Reviewed-by: Ivan Komissarov <ABBAPOH@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>