Rework KVM demo configuration

Rework KVM demo configuration

Changes:
- Move KVM demo configuration and image recipes to a new sub-layer,
  meta-agl-kvm-demo, to keep things that may not be buildable with
  just the agl-demo feature separate to avoid confusion.  It will
  hopefully also avoid clutter in recipes-platform/images.
  This sub-layer is pulled in when the agl-kvm setup feature is
  given to aglsetup.sh.
- Remove the agl-kvm-host-kuksa and agl-kvm-host-audio setup
  features.
- Add new *-guest and *-guest-preconfigured flavors of the Flutter
  IVI and IC images under meta-agl-kvm-demo that that have the
  desired configuration changes baked in.
- Add required qemu-config recipe variants for the new guest image
  flavors. At the moment there is more duplication of configuration
  for this than is desired, and some reworking of agl-qemu-runner
  configuration may come as a follow up to avoid this.
- Remove qemu-config recipe variants for unused Qt guest images.
  If it becomes desirable to use the Qt demo images as guests this
  can be revisited.
- Added agl-kvm-demo-flutter-preconfigured image variant that
  supports the full demo setup (i.e. "green machine" with steering
  wheel, equivalent to CES 2024 demos).

NOTES:
- The agl-kvm-demo image remains and builds roughly the same image
  as before, with the KUKSA.val databroker running in the IVI guest
  and cluster support enabled. Replacing this image with a bbclass
  abstraction and an e.g. agl-kvm-demo-flutter image is under
  consideration.

Bug-AGL: SPEC-5138

Change-Id: I64936208fd032e5ba47366e3a7ff572dc18338e4
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29905
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>