Cherry-pick for CLOUDSTACK-4481

[Min Chen]

Hi Animesh,
        
        Event framework merge (e7a554fc6a23a49949c2d88d6ef680682c6f6bc4)
introduced a regression in ListEventTypes api, which will return nothing.
Commit f501c7bbe1ba3fca4071daa520230a34323900cb is checked into
4.2-forward to fix this bug. Please cherry-pick the commit to 4.2.

        Thanks
        -min