core: Create default disk profiles for data domains

All data domains should have default disk profiles with unlimited QoS
(currently implemented by null)

Otherwise all existing data domains needs manual intervention for
creating disks if the data center is upgraded to version 3.5

Change-Id: I3ee3174e5c7bef3bcd5591b9cd99ff5324bee488
Bug-Url: https://bugzilla.redhat.com/1145241
Signed-off-by: Roy Golan <rgolan@redhat.com>