KVM demo updates

Changes:
- Fix native-shell-client systemd unit to work with the changes
  made for SPEC-4714 (separate agl-compositor systemd unit,
  running as agl-driver).
- Update agl-qemu-runner.sh for the SPEC-4714 changes.
- Tweak cluster guest configuration to use just 2 cores and 1 GB
  of memory to free up resources for the host.
- Add inverted psplash configuration that is pulled in for the
  KVM demo image when building with agl-demo-preload.  This
  matches the inverted screen in the demo hardware.

Bug-AGL: SPEC-4974

Change-Id: Ie57a3550c8c8b12fd64ee14a9f1fbf4a4c40da32
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29460
ci-image-boot-test: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account