webadmin: Coverity defect CID 1230278 (FB.BC_UNCONFIRMED_CAST)

Swapped unsafe cast for a properly-parameterized event listener.

Change-Id: Ied30d82a0ae72acecf504d3307ae1a9f7f35c220
Signed-off-by: Lior Vernia <lvernia@redhat.com>