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 a third patch following
http://gerrit.ovirt.org/#/c/21838/

Change-Id: Ie4ded6654051a23f5abb1bdfb7cf6a142bdfd78a
Signed-off-by: Alissa Bonas <abonas@redhat.com>