[virtio-mem] Adding blank device

[fabianpatras]

Changes

This marks the start of adding support for virtio-mem devices in Firecracker, also called Memory Devices. More details about Memory Devices can be found in Virtual I/O Device Version 1.2, Chapter 5.15.

This PR contains the following:

• new virtio-mem device that is recognised the guest kernel (if the kernel is configured to support virtio-mem), but offer not backend functionality (it does not provide memory),
• a change to microvm-kernel-x86_64-5.10.config to support virtio-mem (to facilitate testing),
• a new file config_vm used to test this device by adding --config-file config_vm to firecracker command, as right now the only way to configure this device is through the config file, no API endpoints are implemented yet

Reason

Adding this Virtio Device aims to improve multi-tenancy by facilitating memory oversubscription.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

PR Checklist

• [x] All commits in this PR are signed (git commit -s).
• [ ] If a specific issue led to this PR, this PR closes the issue.
• [x] The description of changes is clear and encompassing.
• [ ] Any required documentation changes (code and docs) are included in this PR.
• [x] New unsafe code is documented.
• [x] API changes follow the Runbook for Firecracker API changes.
• [ ] User-facing changes are mentioned in CHANGELOG.md.
• [x] All added/changed functionality is tested.

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• [ ] This functionality can be added in rust-vmm.