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 second patch following
http://gerrit.ovirt.org/#/c/21797/

Change-Id: I3ddb7d18ba1758c137dc92c44d5082837cebd18f
Signed-off-by: Alissa Bonas <abonas@redhat.com>