Document VM storage performance options

[abonillabeeche]

Documents the VM storage performance options added to the Harvester UI in harvester/harvester-ui-extension#1133 (harvester/harvester#11550).

Adds a new page, Advanced > VM Storage Performance Options (docs/advanced/vm-storage-performance.md), and a short Storage performance options section in docs/vm/create-vm.md that links to it. Both are also added to the v1.9 docs, since the feature targets v1.9.1.

The page explains what each setting does, its trade-offs, and its recommended use:

• the performance profiles: Default (Balanced), High Performance, and Custom
• Cache Mode: default, none, writeback, and writethrough, including the guest flush requirement for writeback and the direct I/O requirement for none
• I/O Mode: default, native, and threads, including the rule that native requires cache mode none
• Dedicated I/O Thread: purpose, virtio-only requirement, and its effect on the I/O threads policy
• Virtio Block Multi-Queue: the queue count follows the vCPU count automatically, and why counting queues inside the guest is not a reliable check
• I/O Threads Policy: disabled, shared, auto, and supplemental pool, including that supplemental pool adds CPUs to the pod of the virtual machine
• a YAML example of the resulting KubeVirt fields

The descriptions are based on the KubeVirt user guide, the QEMU and libvirt documentation, and behaviour verified on Harvester v1.9.0 with KubeVirt 1.8.4. The same behaviour is covered by the robot suite in harvester/tests#2922.

Dependencies

• The v1.9 page is accurate once feat: expose KubeVirt high-performance storage features in VM and Volume UI harvester-ui-extension#1133 is backported for v1.9.1. It is currently merged to main only.
• The caution about io: native with a different cache mode describes the current behaviour (the virtual machine is accepted but fails to start). If webhook: reject native I/O with a host cache mode harvester#11806 merges, that combination is rejected at admission instead, and I will update the caution accordingly.

No screenshots yet. Images of the expanded per-disk section and the VM-wide section would help after the profile table and in the VM-wide section.

[abonillabeeche]

@Vicente-Cheng @asettle could you please review this? It documents the VM storage performance options from harvester/harvester-ui-extension#1133 (harvester/harvester#11550), with a new Advanced page that explains what each setting does, its trade-offs, and its recommended use. Thanks!