core: adding disk_id as a secondary order-by column

Disks search default order by column is disk_alias.
Since disk alias is not unique, the search query might
return disks in a different order when they are manipulated
(e.g. status changes). Hence, in order to prevent unpredicted
order, added disk_id as a secondary order-by column.

Change-Id: I7e62b44cab4065c41dc7273fc527415b9fba002f
Bug-Url: https://bugzilla.redhat.com/953154
Signed-off-by: Daniel Erez <derez@redhat.com>