design-proposal: instancetype.kubevirt.io - Support declarative VirtualMachine management

[lyarwood]

What this PR does / why we need it:

At present a VirtualMachine using instance types and/or preferences will have runtime derived data such as the name of a ControllerRevision capturing the state of each resource mutated into the core spec during submission.

This breaks declarative management of VirtualMachines as an owner has no way of pre-populating these runtime derived values and will always see changes made to the spec of their VirtualMachine after submission.

This design proposal aims to enable declarative management of VirtualMachines using instance types and/or preferences by using status to track runtime derived data while retaining all existing behavior and lifecycle support of a VirtualMachine using an instance type and/or preference.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

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[0xFelix]

Great idea!

[0xFelix]

It will lead to slightly different behavior of a restored VM. On the original VM the inference can be retriggered, on the restored VM the instancetype/preference will be fixed. Is there a way to keep inferFromVolume and only restore the status?

[lyarwood]

We could always move name, kind, RevisionName and controllerRevisionRef into status during restore when inferFromVolume is provided? That's simple enough and would allow the restored VM to retain the same inferFromVolume behaviour.

[lyarwood]

Actually that doesn't work, we can't change the spec after submission at all even in this case. I guess if the user wants inferFromVolume to work again this case they will need to clear the other fields in the spec and then use the refresh subresource API etc to infer things once again.