Add support for nested virtualization on GCE instances

[jwmay2012]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Adds support for enabling nested virtualization on GCE instances via the enableNestedVirtualization field in GCPMachineSpec. This maps directly to GCP's AdvancedMachineFeatures.EnableNestedVirtualization API field.

This enables running VMs inside GCE instances for container sandboxes, KVM, QEMU, etc.

Special notes for your reviewer:
This is a simple boolean field that maps directly to the GCP API. The field must be set at instance creation time and cannot be changed after creation (GCP limitation).

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Add support for nested virtualization on GCE instances. `GCPMachine.Spec.EnableNestedVirtualization` allows running VMs inside GCE instances.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @jwmay2012. Thanks for your PR.

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[justinsb]

I was wondering if we should nest this under advancedMachineFeatures to reflect the GCP API, but to be honest I'm not sure that "advanced" is really a semantic grouping, so I think I prefer the flattened structure. It's always risky to change the structure because we can't predict the future evolution of the upstream API, but in this case I think it's fine!

[justinsb]

I think we generally treat machines as immutable anyway - i.e. we would launch a new machine, even if the field was mutable. My understanding could be out of date though.

Not sure there's any harm in the NOTE though, but I don't think it's going to be a problem in practice

[justinsb]

/ok-to-test

[jwmay2012]

/retest