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.

[chrisjsimpson]

+1

Publish a kernel + initrd combination on the image server (x86_64 and aarch64)

Yes, if I'm reading issue correctly a user with the following qemu experience:

qemu-system-x86_64 -enable-kvm -kernel bzImage -initrd rootfs.cpio.gz --append "console=ttyS0 init=/init -nographic

Woud be able to publish/upload their kernel and initrd (bzImage / rootfs.cpio.gz) and run.

For those images, rather than allocate a block volume, just stick with the single-volume as is normally done for containers

Consider having the option to have no volume at all (think of a busybox type scenario where all is in ram and a distro is never reaches pivot_root). imho most users would expect a volume by default- having the option of none is useful as less to tidy up if not needed.

A more complex use case might be how to translate networking expectations from a user -> incus e.g.

qemu-system-x86_64 -enable-kvm -kernelbzImage -initrd rootfs.cpio.gz --append "console=ttyS0 init=/init ip=192.168.1.2:192.168.1.1:192.168.1.1:255.255.255.0::eth0:off" -nographic \
  -netdev tap,id=net0,ifname=tap0,script=no,downscript=no\
  -device e1000,netdev=net0

Fantastic live demo by the way!