Fix KVM checkbox when switching from other OS/Release to Ubuntu 18.04.

[Caleb-Ellis]

Done

• Fixed KVM checkbox being disabled when switching from any other OS/Release back to Ubuntu 18.04 if 18.04 is the first release in state.

QA

• Make sure Ubuntu 18.04 is the earliest release available and you have at least 2 releases that aren't Ubuntu (bolla is set up this way).
• Select deployable machines and open the deploy form
• Change selection to Ubuntu 18.04 if not the default, and check that the KVM checkbox is enabled.
• Change the OS and release then change back to Ubuntu
• Check that the checkbox is enabled

Fixes

Fixes canonical-web-and-design/MAAS-squad#1904

[huwshimi]

I can't quite figure out why this is in a useEffect and not on the field's onChange. Is there a reason? If so, maybe a comment would be useful here for future developers (probably me) wondering the same thing.

[Caleb-Ellis]

Basically because the list of releaseOptions is determined by the hook:

const releaseOptions = useSelector((state) =>
  generalSelectors.osInfo.getOsReleases(state, values.oSystem)
);

And since you can't have hooks in callbacks I've moved the logic up into the main component. I guess I could just have get all releaseOptions and then filter based on values.oSystem outside of the hook instead though.