Currently I manually poweroff each KVM guest before rebooting the host. However, this is getting old and now I am up to about 20 guest machines.
Can I trust Ubuntu Server 12.04 to do the Right Thing and hold off on completing a shutdown until the last KVM client is off?
Alternatively is there an easier way to shut all the guests off gracefully?
Yes, this is done automatically, according to this fixed bug: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/350936
See also: https://serverfault.com/questions/200751/can-kvm-suspend-or-shutdown-guests-automatically-on-host-shutdown