image: add kubevirt image build

[ader1990]

As Ignition supports KubeVirt, add a custom oem for it and also the required parts to be able to build an image in .qcow2 format and .gz compression.

Fixes: flatcar/Flatcar#1358

[ader1990]

Tested locally on my Kubevirt Environment and the image works as expected.

Workflow:

1. Deply Kubernetes with CDI and KubeVirt (not the scope of this how to)
2. Use virtctl to create a PVC from the image:

virtctl image-upload pvc flatcar-kubevirt-pvc --size=10Gi --image-path flatcar_production_kubevirt_image.qcow2

3. Create KubeVirt VM

apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
  generation: 1
  labels:
    kubevirt.io/os: linux
  name: vm-flatcar
spec:
  running: true
  template:
    metadata:
      creationTimestamp: null
      labels:
        kubevirt.io/domain: vm-flatcar
    spec:
      architecture: amd64
      nodeSelector:
        kubernetes.io/arch: amd64
      domain:
        cpu:
          cores: 2
        devices:
          disks:
          - disk:
              bus: virtio
            name: disk0
          - cdrom:
              bus: sata
              readonly: true
            name: cloudinitdisk
        machine:
          type: q35
        resources:
          requests:
            memory: 1024M
      volumes:
      - name: disk0
        persistentVolumeClaim:
          claimName: flatcar-kubevirt-pvc
      - cloudInitConfigDrive:
          userData: |
            {"ignition":{"version":"3.3.0"},"kernelArguments":{"shouldExist":["flatcar.autologin"]},"passwd":{"users":[{"name":"core","passwordHash":"$6$sn3ZSJJJln5JkAZb$VDTKzLpCyjlEe7Kh0DKjOnEawkkOoi0tOKVbcCv0FIWSf3u9Y1p1I5YdJJ5L8uDmmMvO2CBlmJZNdxFuekjjE1"}]}}
        name: cloudinitdisk

4. Connect to VM using core/foo password

[github-actions]

Build action triggered: https://github.com/flatcar/scripts/actions/runs/8890177334

[ader1990]

@pothos thank you for the comments.

I have created the PR to enable hostname set using the metadata from the config drive:
flatcar/bootengine#95

[ader1990]

I will also start a documentation PR with the discussion resume on how Ignition or coreos-cloudinit works and in what scenarios, plus the above examples / steps to run Flatcar with KubeVirt.

[pothos]

Thanks

[ader1990]

Tested locally on KubeVirt using Ignition and Bash userdata, and all works as expected.