libvirt: add --bind-storage-ro support for bootc upgrades

[cgwalters]

Implement --bind-storage-ro flag for bcvk libvirt run to enable bootc upgrade workflows from persistent VMs by mounting host container storage read-only.

Assisted-by: Claude Code

[gemini-code-assist]

Summary of Changes

Hello @cgwalters, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the bcvk libvirt run command by introducing a --bind-storage-ro flag. This flag enables the creation of virtual machines with read-only access to the host's container storage via virtiofs, specifically addressing the need for streamlined bootc upgrade processes within persistent VM environments. The changes include modifications to the libvirt domain builder, command-line option parsing, and comprehensive integration tests, along with updated documentation.

Highlights

• New --bind-storage-ro flag: Introduces a new command-line option for bcvk libvirt run to mount host container storage read-only into a VM.
• bootc upgrade support: Designed to facilitate bootc upgrade workflows by providing persistent VMs with direct access to host container images.
• Virtiofs integration: Implements virtiofs for sharing the host's container storage with the guest VM, including necessary libvirt XML configuration.
• Comprehensive testing: Adds an end-to-end integration test to validate the --bind-storage-ro functionality, covering domain creation, XML verification, and in-VM mounting.
• Documentation updates: Updates user documentation and man pages with details and examples for the new --bind-storage-ro flag.

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[gemini-code-assist]

Code Review

This pull request introduces a --bind-storage-ro flag to bcvk libvirt run, enabling bootc upgrade workflows by mounting the host's container storage read-only inside the VM. The implementation adds the necessary libvirt domain configuration for a virtiofs filesystem, along with a comprehensive integration test and documentation updates. My review focuses on ensuring the read-only constraint is correctly enforced, improving test robustness, and aligning the documentation with the implementation. I've found a critical issue where the read-only flag is not being correctly translated into the libvirt XML, and have provided suggestions to fix this and enhance the corresponding test.

[jeckersb]

LGTM, we went over this live earlier