If a KVM virtual machine has not been properly shutdown, it may wait forever at the boot up kernel selection screen for user input. You won’t see any output when trying to virsh console into the VM from the host. You have to fire up a remote VNC session in order to press enter.
In Ubuntu Server you can override this behaviour by adding the following line to /etc/default/grub:
GRUB_RECORDFAIL_TIMEOUT=2
Don’t forget to run update-grub afterwards.