modules-test: fix const cast

Add a missing cast; this fixes a build failure with --enable-modules.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
Message-id: 1566495734-23297-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>