Add Known Issues page: Flatcar Ignition failure on KubeVirt 1.6.x

[mihiragrawal]

What this PR does / why we need it

Adds a Known Issues page to the Kubermatic Virtualization docs, documenting
the KubeVirt 1.6.x defect that breaks Flatcar worker-node provisioning.

On KubeVirt v1.6.4 and newer, Flatcar worker VMs boot and get an IP but never
receive their Ignition configuration, so the node never joins the user cluster.
KubeVirt's PCIe-hotplug port reservation re-defines the libvirt domain a second
time, and that XML round-trip drops the qemu:commandline -fw_cfg opt/com.coreos/config argument that carries Flatcar's Ignition. Ubuntu
(cloud-init / noCloud) is unaffected. This is an upstream KubeVirt bug, not a
Kubermatic Virtualization or machine-controller defect.

The page documents the problem, root cause, the workaround (the
kubevirt.io/placePCIDevicesOnRootComplex annotation — set automatically for
Flatcar by kubermatic/machine-controller#2057, with a manual MachineDeployment
fallback), the Flatcar-only scope, the trade-off (disables unused PCIe hotplug),
and upstream references.

Related

• Workaround: kubevirt: pin Flatcar VMs to PCI root complex to preserve Ignition machine-controller#2057
• Upstream root cause: [sidecars] XML round-trip via xml.Unmarshal/xml.Marshal drops or corrupts namespace-scoped domain sections (qemu:commandline/qemu:arg) kubevirt/kubevirt#16901
• Upstream fix (open): sidecar hooks: preserve QEMUCmd across XML round-trip kubevirt/kubevirt#18460

[archups]

Please take care of review comments for workaround section.

Also please add known issue under KKP docs as well to main and supported releases.

/hold until MC release is out. And make necessary changes in the workaround to include how to pin to the MC release containing the fix until it is available as part of KKP release using following change in KubermaticConfiguration

spec:
  userCluster:
    machineController:
      imageTag: v1.xx.xx

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please ask for approval from archups. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment