tests/docker: change tag naming scheme of our images

tests/docker: change tag naming scheme of our images

We've been misusing the tag naming scheme for some time by overloading
the post : section with the image type. Really it should be saved for
the revision of that particular build. Move the details to the other
side so we have:

  qemu/image-name

with the implied :latest version added by the tooling.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200701135652.1366-18-alex.bennee@linaro.org>