[Merge] ~enr0n/ubuntu-release-upgrader:lp.1960255 into ubuntu-release-upgrader:ubuntu/main

[Nick Rosbrook]

> A brief google search seems to indicate that these processors are commonly
> referred to as POWER8 and POWER9, so we should use that same formatting.
> 
> Thanks for adding a test too, that's great.

I have updated the formatting to follow that convention.

I also tested this on a POWER8 system running impish to confirm the patch works as expected.
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