blk: add methods to retrieve the backing object from `StdIoBackend`

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Summary of the PR

Add three methods to retrieve the inner B from an StdIoBackend. This is useful to inspect or save the inner state of the backend object.

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [ ] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

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• [ ] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).

Left this unchecked because there is no changelog for virtio-blk.

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Rebased on main

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CI failure should be unrelated.