avocado-system-* CI jobs are unreliable

Description of problem

The avocado-system-* CI jobs fail randomly: https://gitlab.com/qemu-project/qemu/-/jobs/5058610614
https://gitlab.com/qemu-project/qemu/-/jobs/5058610654
https://gitlab.com/qemu-project/qemu/-/jobs/5030428571

I don't know how to interpret the test output. Until these CI jobs pass reliably it won't be possible for me to identify when a subtest that is actually healthy/reliable breaks.

Please take a look at the logs and fix or remove unreliable test cases.

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