vhost-user-gpu: fix memory disclosure in virgl_cmd_get_capset_info (CVE-2021-3545)

Otherwise some of the 'resp' will be leaked to guest.

Fixes: CVE-2021-3545
Reported-by: Li Qiang <liq3ea@163.com>
virtio-gpu fix: 42a8dadc ("virtio-gpu: fix information leak
in getting capset info dispatch")

Signed-off-by: Li Qiang <liq3ea@163.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210516030403.107723-2-liq3ea@163.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>