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 #8 following
http://gerrit.ovirt.org/#/c/21975/

Change-Id: I556c524fa99138e3172c635ac619bd8fba39eefd
Signed-off-by: Alissa Bonas <abonas@redhat.com>