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

Closes: #8

[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 adding a --bind-storage-ro flag, which allows users to securely mount the host system's container storage into a virtual machine as a read-only virtiofs filesystem. This capability is essential for enabling seamless bootc upgrade operations within persistent VMs, ensuring that the VM can access required container images without compromising the host's storage integrity.

Highlights

• New --bind-storage-ro flag: Introduced a new --bind-storage-ro flag for the bcvk libvirt run command, enabling read-only mounting of host container storage into virtual machines.
• Enables bootc upgrade workflows: This feature is designed to facilitate bootc upgrade workflows from persistent VMs by providing necessary read-only access to host container images.
• Virtiofs integration: Implemented VirtiofsFilesystem struct and DomainBuilder methods to generate appropriate libvirt XML configurations for virtiofs mounts, including shared memory backing and read-only attributes.
• Comprehensive integration testing: A new end-to-end integration test (test_libvirt_bind_storage_ro) has been added to verify the functionality, covering VM creation, XML validation, SSH access, and enforcement of the read-only mount.
• Documentation updates: The libvirt-run documentation and its man page have been updated to reflect the new flag, its usage, and the benefits of container storage integration.

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[gemini-code-assist]

Code Review

This pull request introduces support for mounting host container storage as read-only within libvirt VMs, a valuable feature for bootc upgrade workflows. The implementation is well-structured, encompassing a new CLI flag, modifications to the libvirt domain XML generation, and the addition of a comprehensive integration test. My review has identified a few areas for improvement, including a bug in the new integration test, a misleading error message, and several opportunities to enhance code maintainability and performance. The accompanying documentation updates are clear and effectively explain the new functionality.

[cgwalters]

      error: unsupported configuration: virtiofs does not yet support read-only mode

Ah yes, old libvirt on this Ubuntu version. Well, good job CI at least.

[cgwalters]

Closing in favor of #20 which comes from my fork