overlay: Add code to verify interactive login for Live ISO

This is similar to: coreos/ignition-dracut#146

For our test system, it generally works really well to inject
things via Ignition.  That PR was about handling failures in the
initramfs *before* Ignition runs.

This PR is trying to help us test the scenario where no Ignition
is injected into the Live ISO.  Let's also use the virtio-channel
approach.

overlay.d/05core/usr/lib/systemd/system-preset/40-coreos.preset

@@ -13,3 +13,5 @@ enable afterburn-firstboot-checkin.service
 enable [email protected]
 # Update agent
 enable zincati.service
+# Testing aid
+enable coreos-liveiso-success.service

overlay.d/05core/usr/lib/systemd/system/coreos-liveiso-success.service

@@ -0,0 +1,17 @@
+# This is used by our test framework in coreos-assembler
+# since we don't have a way to inject testing configuration
+# via Ignition to test this scenario.
+[Unit]
+Description=CoreOS Live ISO virtio success
+Documentation=https://github.com/coreos/fedora-coreos-config
+ConditionPathExists=/run/ostree-live
+ConditionVirtualization=|kvm
+ConditionVirtualization=|qemu
+
+[Service]
+Type=simple
+# See https://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-messages.h
+ExecStart=/bin/sh -c 'journalctl -b -o cat -u systemd-logind.service MESSAGE_ID=8d45620c1a4348dbb17410da57c60c66 | head -1 >dev/null; echo coreos-liveiso-success > /dev/virtio-ports/coreos.liveiso-success'
+
+[Install]
+WantedBy=multi-user.target