I'm happy to discover that ovirt-engine is clean compiled on using jdk9. I used the official JDK9 container to run it from the repo directory:

docker run -it --rm --name ovirt-engine-compile-jdk9 -v "$PWD":/usr/src/mymaven -w  /usr/src/mymaven  docker.io/maven:3-jdk-9 ln -s /etc/java-9-openjdk /usr/lib/jvm/java-9-openjdk-amd64/conf ; mvn clean package