Add support for userspace MSR handling

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Summary of the PR

Add support for userspace MSR handling. This is done by enabling the KVM_CAP_X86_USER_SPACE_MSR capability, which enables two new exits, KVM_EXIT_X86_RDMSR and KVM_EXIT_X86_WRMSR. This allows a VMM to intercept reads and writes to invalid MSRs instead of having KVM automatically inject a #GP into the guest.

This PR adds safe wrappers around KVM_CAP_X86_USER_SPACE_MSR, KVM_EXIT_X86_RDMSR and KVM_EXIT_X86_WRMSR, some helper types and 2 relevant tests.

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.
• [X] 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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Reordered Cap::X86UserSpaceMsr to keep a consistent ordering.

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Rebased on main, fixed conflicts and updated coverage.

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Rebased on main and fixed conflicts.

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Gentle ping for reviewers