hw/display/vhost-user-gpu.c: fix vhost_user_gpu_chr_read()

hw/display/vhost-user-gpu.c: fix vhost_user_gpu_chr_read()

fix vhost_user_gpu_chr_read() where `size` was incorrectly passed to `msg->flags`.

Fixes: 267f664658 ("hw/display: add vhost-user-vga & gpu-pci")
Signed-off-by: Haoran Zhang <wh1sper@zju.edu.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>