Disk: Fix bug when Partition reported twice - Fixes #210

[PlagueHO]

Pull Request (PR) description

• Fix bug when multiple partitions with the same drive letter are reported by the disk subsystem - Fixes Issue #210.

This Pull Request (PR) fixes the following issues

• Fixes #210

Task list

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

@johlju - would you mind reviewing this one for me?

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This change is