feat(vm): add system migration policy

[LopatinDmitr]

Description

Add a system-level live migration policy override sourced from ModuleConfig/virtualization annotation virtualization.deckhouse.io/system-migration-policy.

The controller now reads this annotation at startup and, when valid, applies it globally in live migration policy calculation.

What is the expected result?

1. Set annotation on ModuleConfig/virtualization:
   virtualization.deckhouse.io/system-migration-policy: <valid policy>.
2. Restart/rollout virtualization-controller.
3. Run VMOP migration/eviction.
4. Confirm effective migration configuration follows the system policy override (VM spec and VMOP force do not override it).
5. If annotation is missing or invalid, behavior remains unchanged.

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: core
type: feature
summary: "Add a system live migration policy override via ModuleConfig annotation for VMOP/live migration policy calculation."