feat(vm): add VNC screenshot subresource

[fl64]

Description

Adds a virtualmachines/screenshot subresource to the aggregated virtualization apiserver. It returns a PNG snapshot of the VM graphical console. Under the hood it proxies a plain GET to the KubeVirt vnc/screenshot endpoint (which already ships in our virt-api), so no node-side changes are required. The moveCursor query parameter is supported to wake a blanked screen before the frame is captured.

Why do we need it, and what problem does it solve?

There was no way to grab a still image of a VM screen through the platform API — users had to open an interactive VNC session just to see the current display state. A screenshot subresource lets tools and UIs fetch the current screen as an image (e.g. to show a VM preview or to diagnose a VM that is stuck at a boot/login screen) without a full VNC session.

What is the expected result?

On a running VM:

GET /apis/subresources.virtualization.deckhouse.io/v1alpha2/namespaces/<ns>/virtualmachines/<name>/screenshot

returns Content-Type: image/png with the current screen. Adding ?moveCursor=true nudges the pointer first so a blanked display wakes up. Access is granted by the existing access_remote_display use-capability.

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: vm
type: feature
summary: "Added a screenshot subresource that returns a PNG image of the VM screen."