libvirt: Add support for --transient

[cgwalters]

The use case here is ephemeral VMs for CI tests; we don't want to leak them across host updates and it's more efficient to not create persistent disks for them.

[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 introduces a significant enhancement to the libvirt run command by adding support for --transient virtual machines. This feature allows users to provision short-lived, non-persistent VMs, which are particularly useful for automated testing environments like CI pipelines. By leveraging temporary disk overlays and virsh create for transient domains, the system efficiently manages these ephemeral instances, ensuring they are automatically cleaned up upon shutdown without leaving behind persistent state.

Highlights

• Transient VM Support: Introduced a --transient flag to the libvirt run command, enabling the creation of ephemeral virtual machines.
• Ephemeral VM Use Case: These transient VMs are designed for CI tests, avoiding persistent disk creation and ensuring they do not leak across host updates.
• Integration Test: A new integration test, test_libvirt_transient_vm, has been added to validate the functionality of transient VMs, including verification of non-persistence and proper disk handling.
• Libvirt Domain Configuration: The DomainBuilder now supports a transient_disk option, which adds a <transient/> element to the VM's XML definition for disk configuration.
• Optimized Disk Handling: For transient VMs, the system now directly uses the base disk with a temporary overlay instead of cloning it, improving efficiency.
• Dynamic VM Lifecycle Management: The libvirt run command differentiates between transient and persistent VMs, using virsh create for transient VMs (which are created and started in one step and disappear on shutdown) and the traditional virsh define followed by virsh start for persistent ones.

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[gemini-code-assist]

Code Review

This pull request introduces support for transient VMs in libvirt by adding a --transient flag. When used, it creates a non-persistent VM that is removed on shutdown, which is useful for ephemeral CI environments. The implementation correctly uses virsh create for transient domains and skips cloning the base disk, instead relying on libvirt's overlay capabilities. The changes are well-structured and include a comprehensive integration test.

My review includes a few suggestions to improve robustness and maintainability:

• Refactoring duplicated error-handling logic for virsh commands into a helper.
• Improving the new integration test by using a proper XML parser instead of string matching to make it less brittle.
• Replacing a fixed sleep in the test with a more reliable polling mechanism to avoid flakiness.