hw/virtio/virtio-gpu: Fix compiler warning when compiling with -Wshadow

hw/virtio/virtio-gpu: Fix compiler warning when compiling with -Wshadow

Avoid using trivial variable names in macros, otherwise we get
the following compiler warning when compiling with -Wshadow=local:

In file included from ../../qemu/hw/display/virtio-gpu-virgl.c:19:
../../home/thuth/devel/qemu/hw/display/virtio-gpu-virgl.c:
 In function ‘virgl_cmd_submit_3d’:
../../qemu/include/hw/virtio/virtio-gpu.h:228:16: error: declaration of ‘s’
 shadows a previous local [-Werror=shadow=compatible-local]
  228 |         size_t s;
      |                ^
../../qemu/hw/display/virtio-gpu-virgl.c:215:5: note: in expansion of macro
 ‘VIRTIO_GPU_FILL_CMD’
  215 |     VIRTIO_GPU_FILL_CMD(cs);
      |     ^~~~~~~~~~~~~~~~~~~
../../qemu/hw/display/virtio-gpu-virgl.c:213:12: note: shadowed declaration
 is here
  213 |     size_t s;
      |            ^
cc1: all warnings being treated as errors

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20231009084559.41427-1-thuth@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>