core: Restore storage_pool_id to storage_domain_for_search

In commit a7caea90, data center uuid for storage search was removed.
This fix returns it when the SD is attach to a single DC.

Change-Id: I24cbfe91073f888d58d44b853aca94d0024ef917
Bug-Url: https://bugzilla.redhat.com/1166182
Signed-off-by: Allon Mureinik <amureini@redhat.com>