core: Fix issue with GetStorageDomainByNameQuery

Commit 88756e3897ba467eb50055a7dda0c1afb11295d0 introduced function GetStorageDomainByNameQuery
which faults at vm export. This patch fixes this issue.

Change-Id: Ifae06739c658a1f602421081590790b11bba5806
Bug-Url: https://bugzilla.redhat.com/1087028
Signed-off-by: lzelkha@redhat.com <lzelkha@redhat.com>