core: DB Adding missing check to fix...

DB Adding missing check to fix duplicate key violation

Checking if key exists before attempt to add a record to disks table.
This fixes an attempt to insert duplicate key to the disks table.

Change-Id: Ia1f8befcaad0e20d7259980cb0e5f2e116964245
Signed-off-by: Eli Mesika <emesika@redhat.com>