Fix LUExportInstance

Due to deficiencies in our block device implementation, it is a must to
call SetDiskID on disks before passing them to remote nodes. Since in
export/import, we don't touch the disks themselves, this was not needed
before in this function.

However, since having instance symlinks, the correct ID is needed here
too, and with static minors it's a "must need". This reflects into
failed instance starts after migration and/or failover.

Reviewed-by: ultrotter