fix(disks): restore disk from snapshot when storage rounds size up

[nevermarine]

Description

Restoring a virtual disk (and therefore a virtual machine) from a snapshot fails on storage backends that round volume sizes up to a fixed granularity — for example Ceph RBD rounds up to whole GiB. The restored disk stays in the Failed phase with a ProvisioningFailed / "the specified pvc size is insufficient" error, and any VirtualMachineOperation restore that depends on it never reaches Completed.

It happens when the source disk was created without an explicit size: its volume is requested at the source image's virtual size, but the backend provisions a larger volume, so the snapshot's restore size ends up bigger than the size recorded in the snapshot metadata. On restore that smaller recorded size was rejected as insufficient. It is now treated as a lower bound and raised to the snapshot restore size, but only when the size was not explicitly set by the user (an explicit too-small user request is still rejected).

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

On rounding storage backends (Ceph RBD is the common case) snapshot restore is effectively broken for any disk that was provisioned without an explicit size — the restored disk and the restore operation are stuck in Failed. This restores the ability to recover disks and VMs from snapshots on such storage.

What is the expected result?

Reproduction (on a Ceph RBD storage class):

1. Create a VirtualDisk from a ClusterVirtualImage with spec.persistentVolumeClaim: {} (no size). The PVC is requested at the image virtual size but provisioned rounded up to the next GiB.
2. Create a VirtualDiskSnapshot of it.
3. Create a new VirtualDisk from that snapshot with no explicit size.

Before: the new disk goes to Failed (the specified pvc size is insufficient).
After: the new disk provisions at the snapshot restore size and becomes Ready; VMOP restore completes.

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: disks
type: fix
summary: "Restoring a virtual disk or virtual machine from a snapshot no longer fails on storage that rounds volume sizes up (e.g. Ceph RBD)."