tests/vm: switch centos.aarch64 to CentOS 8 Stream

tests/vm: switch centos.aarch64 to CentOS 8 Stream

Switch this test over to using a cloud image like the base CentOS8 VM
test, which helps make this script a bit simpler too.

Note: At time of writing, this test seems pretty flaky when run without
KVM support for aarch64. Certain unit tests like migration-test,
virtio-net-failover, test-hmp and qom-test seem quite prone to fail
under TCG. Still, this is an improvement in that at least pure build
tests are functional.

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: <20220708153503.18864-5-jsnow@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>