fix(kubevirt): improve unmount for hotplug container disks

[yaroslavborbat]

Description

This PR enhances the unmounting process for hotplugged container disks by introducing improvements that ensure a more reliable and thorough cleanup.

Changes

1. Added the removal of mount point files after unmounting to clean up leftover artifacts.
2. Updated the unmounting logic to rely not only on our managed mount records (stored in files) but also on the actual state of mount points.
   • The implementation now actively checks the real mount points on the system to verify their state and ensures they are properly unmounted.

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

These changes make the unmounting process more robust and help prevent stale mount points from causing issues.

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: kubevirt
type: fix
summary: Improve unmounting process by cleaning up mount point files and verifying actual mount states