fix(vm): preserve spec.blockDeviceRefs order in hotplug handler

[loktev-d]

Description

Iterate over Spec.BlockDeviceRefs slice instead of map when hotplugging to preserve hotplug order.

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

What is the expected result?

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: fix
summary: preserve spec.blockDeviceRefs order in hotplug handler

[deckhouse-BOaTswain]

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