I restarted a standalone KVM host running RHEL6 and when I opened Virtual Machine Manager, my guest virtual machine wasn't listed at all.
Eek!
Turns out I had been messing around with the XML definition in /etc/libvirt/qemu/machinename.xml (I know, I know...consider my hand slapped) and broke it. Fortunately /var/log/libvirt/libvirtd.log was kind enough to let me know what went wrong: