frontend: ignore in findbugs EventArgs unconfirmed cast errors

Fix several excludes that entered to the incorrect filter in patch
http://gerrit.ovirt.org/#/c/21882/

Change-Id: Ifa069c39ec911e0b3c63bea4326f5996f5d4ae8c
Signed-off-by: Alissa Bonas <abonas@redhat.com>