core: remove castings for queries values

Following patch I57b6ce5fc4b5d6bab1481f9bfb9d7861ba09a615 which made the
VdcQueryReturnValue#getReturnValue method to be generic, we can drop
castings for its returned value.

This patch removes those kind of unnecessary castings from the rest of
the packages in the engine that were not changed as part of previous
patch mentioned above.

Change-Id: Ib7b8b9f5721e2efd23cbd92ff77c3d3b5d40d811
Signed-off-by: Arik Hadas <ahadas@redhat.com>