core: Fix NPE in QosValidator

Match behavior to prior refactoring made in
http://gerrit.ovirt.org/#/c/31868

Change-Id: If8143c0e7df22c508e257c7549ecb4f93fdbc525
Bug-Url: https://bugzilla.redhat.com/1136272
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>