symlinks: Add DISK_LINKS_DIR constant

The DISK_LINKS_DIR points to the RUN_DIR/ganeti/instance-disks
directory, which will contain symlinks to the instances' disks. These
provide a stable name accross all nodes for them, and permit
live-migration to happen.

Unfortunately RUN_DIR/ganeti/instance-disks happens to be below ganeti
1.2's BDEV_CACHE_DIR, which will we need to address at some point
(possibly in 2.0).

Reviewed-by: iustinp