Thanks, Karl - interesting stuff about the ballooning thing as well. This: #ps v -u qemu -is this on the vm, yes? I don't have a user qemu so only get a No user name error. Usage has dropped down again now, but I'd be interested to have a look: a script would be handy indeed.
Sorry - That was on the host running kvm. You need to look at your own processes on the vm.