qemu.org hosts git repository mirrors of all submodules.  Update
.gitmodules to use the mirrors and not the upstream repositories.

Mirroring upstream repositories ensures that QEMU continues to build
even when upstream repositories are deleted or temporarily offline.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190425145420.8888-1-stefanha@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>