frontend: ignore in findbugs EventArgs unconfirmed cast errors

Ignore in findbugs errors related to casting EventArgs.
The casting is safe in this case, and a future refactoring
will make findbug warning resolved in any case.(RFE #1034844)
This is the fourth  patch following
http://gerrit.ovirt.org/#/c/21847/

Change-Id: I011253855bddbc34a1c306f6084bd8fc12c154d2
Signed-off-by: Alissa Bonas <abonas@redhat.com>