core: Fix FB on AddExistingFileStorageDomain

This patch reverts 5ccea20c205457f70cbd480f80455f600b386836 and offers a
proper fix for the (bogus) NP_NULL_PARAM_DEREF error in
AddExistingFileStorageDomainCommand.

The domainFromIrs variable can NEVER be null, hence it's meaningless to
protect against it.

Change-Id: Id8c5a11ef872cac6aeebfa18bf28757fb4a8b26c
Signed-off-by: Allon Mureinik <amureini@redhat.com>