support qcow2 overlay for vm with persistance

[adamsch1]

This is an idea

---

Note

Introduces a new overlay volume type enabling qcow2 copy-on-write disks with PVC/HostPath backing and optional persistent targets, with full validation, rendering, conversion, creation, and cleanup paths.

• API:
  • Add v1.OverlayVolumeSource to VolumeSource with backing (backingPVC/backingHostPath), target (targetPVC/targetHostPath), backingFormat, and persistent.
  • Update OpenAPI/Swagger, deepcopy, client-go openapi, validations, and API testdata.
• Validation:
  • Extend VMI create admitter to validate overlay backing/target exclusivity, persistent rules, PVC/HostPath fields, and backingFormat (raw|qcow2).
• Controller (virt-controller):
  • Render overlay volumes: mount backing-<vol> and optional target-<vol> via PVC or HostPath.
• Launcher (virt-launcher):
  • Converter: map overlay to libvirt XML with qcow2 source and proper backingStore (file/block, format).
  • Manager: create overlay images via new handler during domain setup.
• Ephemeral Disk:
  • Add overlay-disk-handler to create overlay images and cleanup non-persistent overlays; tests added.
  • Make image creation idempotent in CreateBackedImageForVolume.
• virt-handler:
  • Cleanup non-persistent overlay files on VMI teardown (best-effort).
• Docs:
  • Add OVERLAY_VOLUMES.md and OVERLAY_VOLUME_STATES.md with usage and state matrix.

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