Add bcvk osbuild-disk that wraps bootc-image-builder.

Support having the container image quay.io/centos-bootc/bootc-image-builder on the host system, same as the bootc image. It's basically the same as to-disk where we use ephemeral and then mount the host's container storage and get output.

However things become a lot more complex to do direct cloud uploads; right now the ephemeral VMs (semi intentionally) have no network at all. I think we'd want to split the bib phase into two parts.

One possible trick here is that bib (when run privileged) itself learns about bcvk and re-spawns itself using that to do the disk generation.