Libvirt backend support

[clarfonthey]

Clear and concise description of the problem

Docker containers offer a majority of the cases needed for builds, but in the case of non-Linux systems like OpenBSD, NetBSD, etc., launching complete virtual machines for builds may be required. We need a new backend to support this.

Suggested solution

Libvirt is probably the most versatile system for doing this, since it provides a common API for running things under several different hypervisors.

Alternative

Without libvirt, we'd probably have to manage multiple hypervisor platforms individually. However, it does require a separate runtime layer, meaning that it does add an extra dependency. If we were to implement these hypervisor backends directly, it wouldn't make libvirt redundant completely, but it would make the need for it less pronounced.

Additional context

Most commercial CI systems allow launching full VMs for their versatility, although it's obviously much slower than using containers. To fully replace these providers, woodpecker would need a VM-based runtime.

Validations

• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[6543]

the biggest question would be, how to handle "images" ... - is there a common way / index for vm images ?!?

[danog]

About this, just found https://github.com/containers/krunvm which can create actual VMs using KVM from plain OCI images!

[namgo]

index for vm images

Libvirt provides no globally available registry but there are storage-pools. https://libvirt.org/storage.html#network-filesystem-pool suggests CIFS/NFS/glusterfs as principally supported network protocols for accessing storage pools, the expectation is a LAN environment.

Neither vagrant nor packer provide a container registry feature. Network-booting a live image might work in some scenarios but I don't feel as though it is standardized enough - hoping I'm wrong.

A hosting organization's servers might provide available golden images to users like you might see in a container registry.

I would expect to provide a series of pools on each libvirt-runner. I would assume "image_libvirt:" would expect "pool/image_name" or simply enumerate pools and select image_name.

Edit: I re-organized thoughts for clarity.

I apologize since I'm new to this repo and don't know all the norms, just checking it out trying to see if it fits my use-case.

Personally I've been looking for a CI/CD system for the Plan9Front Operating System, I suspect the Local backend will be mostly fine for my needs but it will require me to spin up dedicated Plan9 infrastructure; libvirt support would be of great convenience to me!