Implement VM support for OCI images

[stgraber]

Currently OCI images can only be run as containers on top of Incus.
It would make sense to also allow them to run as lightweight virtual machines (--vm flag).

To do so, I expect we'll want to:

• Publish a kernel + initrd combination on the image server (x86_64 and aarch64)
• Have that initrd contain a small loader that will:
  • Mount the root filesystem over virtiofs
  • Re-shuffle the mount table and pivot_root
  • Mount the agent drive(s)
  • Spawn incus-agent
  • Exec into the container entry point ensuring it's attached to /dev/console
• Have Incus treat all OCI images as both container and VM capable
• For those images, rather than allocate a block volume, just stick with the single-volume as is normally done for containers
• On startup, if dealing with OCI, ensure that we have a load kernel and initrd available for it, then bypass the firmware logic and use direct kernel booting to boot the image

This should give us a very lightweight VM which effectively acts almost identically to a traditional OCI container but can still handle all of our normal VM devices and config options.