The 'elem' is allocated memory in vu_queue_pop(), and its memory should be
freed in all error branches after vu_queue_pop().
In addition, in order to free the 'elem' memory outside of while(1) loop, move
the definition of 'elem' to the beginning of vus_proc_req().

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Alex Chen <alex.chen@huawei.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <20201125013055.34147-1-alex.chen@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>