core: Fetch unregisterd entity from all the Storage Domains.

Add the ability to fetch an entity from all the
Storage Domains with a stored procedure, by passing null as storage domain id.

This stored procedure should be used for fetching Tempalte with disk copied on several
Storage Domains.

Change-Id: Ia279818045ee3adfcda6383b010ac9f7e283886f
Bug-Url: https://bugzilla.redhat.com/1138136
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>