Rebooting a KVM host at $WORK seems to take a long time -- as in, a long time to actually reboot the host after I type "shutdown -r now". But then the process list shows this:
root 26881 0.0 0.0 64328 824 ? S 13:30 0:00 /bin/sh /etc/rc6.d/K01libvirt-guests stop
root 26882 0.0 0.0 130220 3504 ? S 13:30 0:00 virsh managedsave 128b38e0-ce1a-eb4b-5ee5-2746cd0926ce
root 26890 0.0 0.0 8716 1084 ? S 13:30 0:00 sh -c cat | { dd bs=4096 seek=1 if=/dev/null && dd bs=1048576; } 1<>/var/lib/libvirt/qemu/save//vm-01.example.com.save
root 26891 1.1 0.0 3808 440 ? S 13:30 0:00 cat
root 26892 0.0 0.0 8716 576 ? S 13:30 0:00 sh -c cat | { dd bs=4096 seek=1 if=/dev/null && dd bs=1048576; } 1<>/var/lib/libvirt/qemu/save//vm-01.example.com.save
And now I understand.
I had a new VM host at $WORK I was trying to set up, and I kept running into problems when installing a VM with koan/cobbler: it would complain that "at most 2047 MB can be simulated" for a VM. This was on a 64-bit machine, with a 64-bit kernel (CentOS 5.8), with 128 GB of memory installed (and detected), with a 64-bit guest being installed.
In the end, I had neglected to install the kvm package from CentOS, which among other things includes the kvm-kmod module. Once I got that installed and rebooted, everything went fine.