feat(vm): support hotplug/unplug block devices via spec.blockDeviceRefs

[loktev-d]

Description

Add support for hot-plugging and unplugging block devices via spec.blockDeviceRefs on a running VM, without requiring a restart.

Key changes:

• New HotplugHandler in VM controller: reconciles spec.blockDeviceRefs against KVVM volumes — hotplugs missing disks via KubeVirt AddVolume and unplugs removed disks via RemoveVolume.
• All disks marked as hotpluggable: ApplyVirtualMachineSpec sets Hotpluggable: true.
• All blockDeviceRefs changes are ActionApplyImmediate: the vmchange comparator no longer requires VM restart for any block device changes (add, remove, replace, reorder).
• New spec.blockDeviceRefs[].bootOrder field: optional field to explicitly control boot priority. If not set, positional order is used.
• Simplified ApplyVirtualMachineSpec: removed ClearDisks + rebuild pattern, removed hotplug snapshot/reapply logic. Block device application extracted into applyBlockDeviceRefs / setBlockDeviceDisk.
• Boot order validation webhook: validates bootOrder >= 1 and rejects duplicate values.

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

Previously, runtime disk management required creating VirtualMachineBlockDeviceAttachment (VMBDA) resources. This PR enables the same capability directly through the VM spec — adding or removing entries in spec.blockDeviceRefs on a running VM will hotplug/unplug disks immediately.

What is the expected result?

1. Add a new entry to spec.blockDeviceRefs on a running VM → disk is hotplugged without restart.
2. Remove an entry from spec.blockDeviceRefs on a running VM → disk is unplugged without restart.
3. Set explicit bootOrder on block devices → VM boots from the correct device.
4. Existing VMs continue to work after upgrade (all disks become hotpluggable).
5. VMBDA-attached disks coexist with spec-attached disks.

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: support hotplug/unplug block devices via spec.blockDeviceRefs

[deckhouse-BOaTswain]

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