restapi: 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 api
code.

Change-Id: I499ca2f08f2c3b9f0ceef0999367b2b9be031890
Signed-off-by: Arik Hadas <ahadas@redhat.com>