core: Set read only in the CDA instead the constructor

Setting read only property in the disk at the CDA phase in the constructor to avoid NPE
when there is no disk in the DB.

Change-Id: Ib70637b4fc29a95bb73aa57a6f3f0c74d4d16ca1
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>