feat: support SEV-SNP Report Version 5 (temporary GitHub dependency)

[hyperfinitism]

This PR adds support for SEV-SNP Report Version 5 by updating the code and bumping the sev crate.

Fixes #113

What's changed

• Bump the sev crate to the latest version
• Modifies the code to support new fields introduced in Report Version 5 (e.g., launch_mit_vector)
• Update README.md to reflect these changes

⚠️ Temporary Measure (as of 7 Aug 2025)

This PR (draft) is a temporary workaround using an unreleased version of sev. Once an official release of the sev crate with Report V5 support becomes available, I will:

• Revert the GitHub dependency in Cargo.toml to a versioned release.
• Regenerate Cargo.lock accordingly.

Summary by Sourcery

Add support for SEV-SNP Report Version 5 by updating the sev dependency, handling the new launch_mit_vector field in key derivation, and updating documentation.

New Features:

• Add --launch_mit_vector option to include the launch mitigation vector in derived key requests
• Bump sev crate to GitHub dependency to enable Report Version 6 support

Build:

• Temporarily switch sev crate in Cargo.toml to GitHub source for unreleased v6

Documentation:

• Update README to document the new --launch_mit_vector flag

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

This PR integrates support for the new launch_mit_vector field introduced in SEV-SNP Report Version 6 by extending the CLI, propagating the new parameter through key derivation calls, updating dependencies to a temporary GitHub release of the sev crate, and aligning documentation accordingly.

Entity relationship diagram for KeyArgs and DerivedKey with launch_mit_vector

erDiagram
    KEYARGS {
        Option u64 tcbv
        Option u64 lmv
    }
    DERIVEDKEY {
        root_key_select
        GuestFieldSelect
        vmpl
        gsvn
        tcbv
        lmv
    }
    KEYARGS ||--o{ DERIVEDKEY : provides

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Class diagram for updated KeyArgs struct and get_derived_key function

classDiagram
    class KeyArgs {
        +Option<u64> tcbv
        +Option<u64> lmv
    }
    class DerivedKey {
        +new(root_key_select, GuestFieldSelect, vmpl, gsvn, tcbv, lmv)
    }
    class Firmware {
        +open()
        +get_derived_key(option, request)
    }
    KeyArgs --> DerivedKey : used in new()
    DerivedKey --> Firmware : passed to get_derived_key()

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File-Level Changes

Change	Details	Files
Introduce launch_mit_vector support in key derivation	Add an optional lmv field to KeyArgs Pass args.lmv into the DerivedKey::new constructor Adjust the key request call to include the new launch mitigation vector parameter	src/key.rs
Bump sev crate to temporary GitHub dependency	Replace the versioned sev entry with a Git URL in Cargo.toml Regenerate Cargo.lock to capture the new Git dependency	Cargo.toml Cargo.lock
Update documentation for the new CLI flag	Extend the key command description with the --launch_mit_vector option Add -l, --launch_mit_vector entry to the CLI options table in README.md	README.md

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#113	Fix the failure of the 'snpguest report --random' command due to 'Skipped bytes were expected to be zeroed' error on SEV-SNP-enabled systems with newer kernels.	✅
#113	Ensure that the snp_report_req structure generated by the --random flag has all reserved bytes zeroed and only user_data populated, matching kernel expectations (including for Report Version 6).	✅
#113	Update the codebase to support SEV-SNP Report Version 6, including updating dependencies and documentation as needed.	✅

Possibly linked issues

• report command fails with "Skipped bytes were expected to be zeroed" #113: The PR updates sev crate and code to support SEV-SNP Report V6, fixing deserialization error.

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[hyperfinitism]

This PR is replaced by #115