Enable image tests on aarch64

[ondrejbudai]

We're currently not running any tests on aarch64 which we should really begin to do. We can use the c6g ec2 instances to do it.

My notes:

• We can boot test AMIs on AWS.
• We cannot boot test qcow2 images because c6g instances (except for metal) don't support (nested) virtualization.

Testing on other alt arches would be nice as well but aarch64 is a good and easy start.

[gicmo]

Was this done via #844?

[lavocatt]

@jrusz I think this is done, do you agree ? If so we can close the issue.

[jrusz]

Yes this looks done.