Ubuntu 14.04 originally shipped with a 4.4-based kernel, which didn’t yet support booting as a KVM guest on GICv3 systems. This meant you could only boot trusty instances on GICv2-based hosts. However, thanks to our Foundations team, Ubuntu 14.04 (‘trusty’)/arm64 images have switched to using the Ubuntu HWE kernel. This means you can now run Ubuntu 14.04 (‘trusty’) KVM guests on GICv3 ARM64 hosts, such as those based on Cavium ThunderX, HiSilicon Hip07 and Qualcomm Centriq.
Ubuntu cloud images are available here. See this page for info on how to boot cloud images directly in QEMU. Note: these images will also just work with the Newton release of OpenStack on Ubuntu – more on that later.