core: Get all unregisterd entities for Storage Domain

When entity type is NULL, fetch all the unregistered entities

Change-Id: I1fa749c309381a9f034dc8e6ea499740c33f8990
Bug-Url: https://bugzilla.redhat.com/1138114
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>