I've been pulling my hair out trying to get Ubuntu 12.04LTS server (64bit) working under a standard KVM/libvirt setup on Ubuntu 14.04.
I can create a new virtual machine using virt-manager and run through the installation process from an ISO image. However, when I reboot on completion the new virtual machine hangs after fsck. On trying repeatedly and fiddling with disk bus options it now gets a little further and hangs on "starting configure network device".
During the install I kept all VM options as vanilla for Ubuntu 12.04 and selected packages for LAMP + SSH server.