Use git archive to create tarballs of qemu and submodules instead of
cloning the repository and the submodules.  This is a order of magnitude
faster because it doesn't fetch the submodules from the internet each
time the script runs.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190520124716.30472-2-kraxel@redhat.com>
[AJB: fixed up tabs]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>