core: Renamed the getStorageDomainId Method

The method's name implies that it's a simple getter, even though it does
not always return the expected storage domain even if it exists
(permissions issues for example).
Thus, renamed to getStorageDomainIdForQuery.

Change-Id: I93e7fdda6ffacb0fc1fd740fe69390864100ab7b
Related-To: https://bugzilla.redhat.com/1085398
Signed-off-by: Idan Shaby <ishaby@redhat.com>