tests/vm: upgrade Ubuntu 18.04 VM to 20.04

tests/vm: upgrade Ubuntu 18.04 VM to 20.04

18.04 has fallen out of our support window, so move ubuntu.aarch64
forward to ubuntu 20.04, which is now our oldest supported Ubuntu
release.

Notes:

This checksum changes periodically; use a fixed point image with a known
checksum so that the image isn't re-downloaded on every single
invocation. (The checksum for the 18.04 image was already incorrect at
the time of writing.)

Just like the centos.aarch64 test, this test currently seems very
flaky when run as a TCG test.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220708153503.18864-6-jsnow@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>