fix(vm): fix display BlockDeviceReady condition's message

[LopatinDmitr]

Description

This pull request refactors the handling of block device conditions within the VM controller.

• Message Generation Simplification:

  • Consolidated message generation for disk states by removing redundant functions and using a closure addUsageMessage to handle single and multiple disk scenarios more concisely.

• Issue Fix:

  • Corrected the issue where a stopped VM with a single disk incorrectly displayed the BlockDevicesReady status as False.

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: Improve BlockDevicesReady's condition message
impact_level: low