We cloned the QEMU repository from the local storage. Since the
submodules are also available there, clone them too. This is
quicker and reduce network use.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[AJB: incorporated review suggestions from danpb]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>