[v3] Help wanted for enabling -Wshadow=local

[Markus Armbruster <armbru@redhat.com>]

Local variables shadowing other local variables or parameters make the
code needlessly hard to understand.  Bugs love to hide in such code.
Evidence: "[PATCH v3 1/7] migration/rdma: Fix save_page method to fail
on polling error".

Enabling -Wshadow would prevent bugs like this one.  But we have to
clean up all the offenders first.

Quite a few people responded to my calls for help.  Thank you so much!

I'm collecting patches in my git repo at
https://repo.or.cz/qemu/armbru.git in branch shadow-next.  All but the
last two are in a pending pull request.

My test build is down to seven files with warnings.  "[PATCH v2 0/3]
hexagon: GETPC() and shadowing fixes" takes care of four, but it needs a
rebase.

Remaining three:

    In file included from ../hw/display/virtio-gpu-virgl.c:19:
    ../hw/display/virtio-gpu-virgl.c: In function ‘virgl_cmd_submit_3d’:
    /work/armbru/qemu/include/hw/virtio/virtio-gpu.h:228:16: warning: declaration of ‘s’ shadows a previous local [-Wshadow=compatible-local]
      228 |         size_t s;                                                       \
          |                ^
    ../hw/display/virtio-gpu-virgl.c:215:5: note: in expansion of macro ‘VIRTIO_GPU_FILL_CMD’
      215 |     VIRTIO_GPU_FILL_CMD(cs);
          |     ^~~~~~~~~~~~~~~~~~~
    ../hw/display/virtio-gpu-virgl.c:213:12: note: shadowed declaration is here
      213 |     size_t s;
          |            ^

    In file included from ../contrib/vhost-user-gpu/virgl.h:18,
                     from ../contrib/vhost-user-gpu/virgl.c:17:
    ../contrib/vhost-user-gpu/virgl.c: In function ‘virgl_cmd_submit_3d’:
    ../contrib/vhost-user-gpu/vugpu.h:167:16: warning: declaration of ‘s’ shadows a previous local [-Wshadow=compatible-local]
      167 |         size_t s;                                               \
          |                ^
    ../contrib/vhost-user-gpu/virgl.c:203:5: note: in expansion of macro ‘VUGPU_FILL_CMD’
      203 |     VUGPU_FILL_CMD(cs);
          |     ^~~~~~~~~~~~~~
    ../contrib/vhost-user-gpu/virgl.c:201:12: note: shadowed declaration is here
      201 |     size_t s;
          |            ^

    ../contrib/vhost-user-gpu/vhost-user-gpu.c: In function ‘vg_resource_flush’:
    ../contrib/vhost-user-gpu/vhost-user-gpu.c:837:29: warning: declaration of ‘i’ shadows a previous local [-Wshadow=local]
      837 |             pixman_image_t *i =
          |                             ^
    ../contrib/vhost-user-gpu/vhost-user-gpu.c:757:9: note: shadowed declaration is here
      757 |     int i;
          |         ^

Gerd, Marc-André, or anybody else?

More warnings may lurk in code my test build doesn't compile.  Need a
full CI build with -Wshadow=local to find them.  Anybody care to kick
one off?