virgl, vtest: Correct the transfer size calculation

The transfer size used in virglrenderer refers to uint32_t, so one
must add 3 and then divide by 4 instead of adding 3/4 which is a no-op
with integers.

Fixes: b3b82fe8 virgl/vtest: add vtest driver

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
(cherry picked from commit 5d7858f1)