Add KubeVirt platform artifact and image information

[rmohr]

Artifact: KubeVirt containerDisks have a normal qcow2 image as base,
similar configured like for OpenStack.

Image: KubeVirt consumes directly a qcow2 disk inside a container which
is described in the images section. It is based on the qcow2 from the
base artifacts for kubevirt.

This schema extension is needed to include informations about containerDisks created in coreos/coreos-assembler#2750.

HyperShift uses this information in rhcos.json to determine which rhcos image to pull for the KubeVirt platform (similart to what it does for AWS).

The final image information looks like this:

                "kubevirt": {
                    "image": "quay.io/openshift-release-dev/rhcos@sha256:67a81539946ec0397196c145394553b8e0241acf27b14ae9de43bc56e167f773"
                }

Building the intaller with this version and a corresponding rhcos.json give the expected results:

$ ./bin/openshift-install coreos print-stream-json | jq -r '.architectures.x86_64.images.kubevirt'
{
  "image": "quay.io/openshift-release-dev/rhcos@sha256:67a81539946ec0397196c145394553b8e0241acf27b14ae9de43bc56e167f773"
}

and

$ ./bin/openshift-install coreos print-stream-json | jq -r '.architectures.x86_64.artifacts.kubevirt'
{
  "release": "33.20201201.3.0",
  "formats": {
    "qcow2.xz": {
      "disk": {
        "location": "https://builds.coreos.fedoraproject.org/prod/streams/stable/builds/33.20201201.3.0/x86_64/fedora-coreos-33.20201201.3.0-kubevirt.x86_64.qcow2.xz",
        "signature": "https://builds.coreos.fedoraproject.org/prod/streams/stable/builds/33.20201201.3.0/x86_64/fedora-coreos-33.20201201.3.0-kubevirt.x86_64.qcow2.xz.sig",
        "sha256": "2be55c5aa1f53eb9a869826dacbab75706ee6bd59185b935ac9be546cc132a85"
      }
    }
  }
}

[rmohr]

/cc @travier
/cc @miabbott

I am unsure if I should also include tags, but digest are most likely preferred.

[rmohr]

/cc @davidvossel

[cgwalters]

Thanks for doing this!

[rmohr]

@cgwalters thanks for the reviews! Do you know when a new release tag will be created?

[bgilbert]

Couple nits. @rmohr, if you can do a quick followup PR, I'll tag a release afterward.

[bgilbert]

Followup in #42.