manager: fix links handling in devperm

Fixup of 89863d6c

For links like this:

$ ls -l /dev/disk/by-path/pci-0000:42:00.0-nvme-1
lrwxrwxrwx 1 root root 13 Oct 25 15:19 /dev/disk/by-path/pci-0000:42:00.0-nvme-1 -> ../../nvme7n1

os.readlink() returns "../../nvme7n1" while we want "/dev/nvme7n1".