vmbus_serial_guest: add tx only option

[tjones60]

Add an option to the VMBUS serial relay that runs in OpenHCL to drop all rx traffic from the host and only allow tx traffic from the guest. This could be attested to in the future and offer a more secure way to debug a VM using the serial console, since only allowing one-way serial traffic would greatly reduce the possible attack surface. This is useful primarily for CVMs that don't have a framebuffer and are usually configured with serial disabled completely.

[github-actions[bot]]

⚠️ Unsafe Code Detected

This PR modifies files containing unsafe Rust code. Extra scrutiny is required during review.

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[github-actions[bot]]

At least one Petri test failed.

[smalis-msft]

Why?

[tjones60]

Why?

This is intended to be an option for CVMs that could be attested to, so you could enable serial output for debugging purposes without allowing interactive login to reduce risk.

[smalis-msft]

Put that in the pr description?

[mebersol]

    console_enabled: dps.general.com1_enabled

an additional runtime claim is needed here.

I think we agreed on something like "console-mode:serial|serialbootdiagnosticsonly"

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Refers to: openhcl/underhill_core/src/worker.rs:1782 in 3d6d329. [](commit_id = 3d6d3295eb026b51fb956576d86cdc8cfc67058e, deletion_comment = False)