Fix issue when VHD file mounted

[nyanhp]

Pull Request (PR) description

This PR fixes an issue with Get/Test and the resource VHDFile. If the VHD that files are copied to/from is attached to a running machine, the resource fails and the system is not in the desired state.

This PR skips mounting the VHD if the machine is running. Documentation has been updated to include that change.

This Pull Request (PR) fixes the following issues

• Fixes #207

Task list

• [x] Added an entry to the change log under the Unreleased section of the file CHANGELOG.md. Entry should say what was changed and how that affects users (if applicable), and reference the issue being resolved (if applicable).
• [x] Resource documentation updated in the resource's README.md.
• [ ] Resource parameter descriptions updated in schema.mof.
• [ ] Comment-based help updated, including parameter descriptions.
• [x] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [x] Unit tests added/updated. See DSC Community Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• [x] New/changed code adheres to DSC Community Style Guidelines.

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This change is