meta-agl-kvm-demo: update for YP scarthgap and clean up

meta-agl-kvm-demo: update for YP scarthgap and clean up

Note that while these changes do get the KVM demo images building
and bootable, some further investigation is required into getting
the guest displays rendering to the outputs.

Changes:
- Move some of the feature template and multiconfig files that were
  accidentally left in meta-agl-demo to meta-agl-kvm-demo.
- Rework how MACHINE is defined for the guest multiconfig to make it
  a bit more obvious how other machines could be supported.  Sadly,
  experimentaion has shown that trying to do a generic
  virtio-${TUNE_ARCH} or similar scheme does not work with BitBake's
  parser, so document that a bit.
- Update agl-qemu-runner.sh for change to IMAGE_NAME_SUFFIX default
  value, and qemu now warning about using "-vga" with the "virt"
  machine which does not support it.
- Add new "agl-virtio-guest" feature to AGL_FEATURES in guest to get
  virtio kernel configuration.

Bug-AGL: SPEC-5201

Change-Id: I194a5c49adf6242cd91d205e19a8eecde698ad7e
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/30102
Tested-by: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
ci-image-build: Jenkins Job builder account