scripts: don't throw away stderr when checking out git submodules

The stderr from git is important if git fails to checkout modules
due to network problems, or other unexpected errors.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20171020130748.22983-1-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
