"git archive" fails when a submodule has a modification, because "git
stash create" doesn't handle submodules. Let's teach our
archive-source.sh to handle modifications in submodules the same way
as qemu tree, by creating a stash.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190708200250.12017-1-marcandre.lureau@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>