Comment # 5 on bug 1009933 from Roger Whittaker

I'm afraid I've destroyed the evidence because I really needed to use the
system.

Before rebuilding it I had looked at "rpm -Va" and also compared package lists
with a clean install and could not find anything wrong.  I hadn't made any
major configuration changes - just added the following to
/etc/libvirt/qemu.conf 

user = "root"
group = "root"
clear_emulator_capabilities = 0

And I was using 9p sharing of files.  But all that is working fine now.

I think this is probably "invalid" but I've no idea why or what happened. 
Everything else on the updated system was working as expected.