A bit more data regarding the disk-write issue ...

- I have determined that the virtual machines do little to no disk I/O, 
and only at infrequent random times.  The host machine and none of the 
VMs do any swapping.
- I tried to use the Virtio flavor of disk, but the BSD-based VMs can't 
use them, so I had to stay with IDE disk emulation.
- It doesn't matter how many virtual machines I have running on the 
host, disk I/O is always once per second.  There's just more of it 
during each burst as the number of VMs increases.

It's really puzzling why libvirtd would be writing to each active disk 
image (as shown by 'iotop') when there is no real reason for it as far 
as the VMs are concerned.

Any ideas why?  Am I missing something really obvious?  (wouldn't be the 
first time for that!)

Chuck