docker: don't rely on submodules existing in the main checkout

When building the tarball to pass into the docker/vm test image,
the code relies on the git submodules being checked out in the
main checkout.

ie if the developer has not run 'git submodule update --init dtc'
many of the docker tests will fail due to the libfdt package not
being present in the test images. Patchew manually checks out the
dtc submodule in the main git checkout, but this is a bad idea.

When running tests we want to have a predictable set of submodules
included in the source that's tested. The build environment is
completely independent of the developers host OS, so the submodules
the developer has checked out should not be considered relevant for
the tests.

This changes the archive-source.sh script so that it clones the
current git checkout into a temporary directory, checks out a
fixed set of submodules, builds the tarball and finally removes
the temporary git clone.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170929101201.21039-3-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
