Speeding up qemu-kvm

> Some block drivers perform badly with cache=writethrough, most notably, qcow2. If performance is more important than correctness, cache=writeback should be used with qcow2.

I switched to cache=unsafe, and life became way better. The kind of tests I do in VMs let me recreate the image if there's a sudden crash, so safety is not such a worry. Not sure if virt-install does something like that.