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 patch #9 following
http://gerrit.ovirt.org/#/c/22089/

Change-Id: Id2fdea041103c13530acd16c812f090942c05ce4
Signed-off-by: Alissa Bonas <abonas@redhat.com>