Implement QEMU autopkgtest virtualization backend

To allow us to run tasks under QEMU, we need to implement the Executor and Instance API. (#219 (closed))

How we envision QEMU instances to work:

• The QEMU executor downloads system-images into the image cache with ImageCache.
• An instance uses a COW overlay image on top of the system-image, stored in a temporary directory.
• Unsafe caching can be used as the VM is ephemeral.
• KVM acceleration is used if possible.
• Instance memory and disk sizes are static, for now.
• Host networking is used, for now.
• Communication with the VM is via QMP and qemu-guest-agent.

Requirements:

• autopkgtest_virt_args should just pass the path to the system-image, on the command line.
• Creating the instance should:
  1. Download the system-image into the image cache, if necessary.
  2. Create a per-instance temporary directory.
  3. Create an overlay image for the instance: qemu-img create -f qcow2 -F $BASE_FORMAT -b $TEMP/overlay.qcow2
• Deleting the instance (including GC) should:
  1. Stop the instance, if it's running
  2. Clean up the temporary directory.
• Starting the instance should:
  1. Start a headless QEMU instance with:
     • The overlay image as rootfs.
     • An EFI BIOS
     • A port-forward from a local port to ssh in the VM (if we need it?)
     • Serial ports bound to unix sockets in the instances' temporary directory.
     • QMP bound to a unix socket in the instance's temporary directory.
     • Start an asyncio driver to connect to the QMP socket.
  2. Watch the serial console and login when a prompt appears.
  3. Install qemu-guest-agent in the VM.
• Stopping the instance should:
  1. Ask QEMU to gracefully shut down, via QMP (guest-shutdown).
  2. Kill QEMU, if it doesn't shut down within 30 seconds, via QMP(quit).
• File push should install a file with appropriate permissions in the VM. This can be done using QMP (guest-file-open, guest-file-write, guest-file-close).
• File pull should copy a file out of the VM. This can be done using QMP (guest-file-open, guest-file-read, guest-file-close).
• Run should execute the provided command with QMP (guest-exec).

Discussion points:

• Any concerns about QMP / guest-agent? I have no experience with QMP, but it looks like a good option.
• Can we require qemu-guest-agent to be installed in the image? That would simplify startup.
• Should we use autopkgtest's autopkgtest_qemu library to implement this, instead of QMP? It does almost exactly what we need, out of the box. (Although some of the useful bits live in autopkgtest-virt-qemu). It requires python3 in the VM and communicates with the VM through files in a virtfs, which seems more sensible than trying to escape stuff for shells, but more complex than QMP.
• Or, do we want to go down this road at all? Incus can drive QEMU for us, and it'll look almost exactly like a container. (#271 (closed))
• Do we want a mechanism to define RAM and disk space? Should this be part of the image, or a parameter to the task?
• Do we want to use bridges for networking? (requiring root) My inclination is to use Incus to drive Qemu instances that need more network control.