To drastically improve virtual drive performance of qemu guests, create images with metadata preallocation:
% qemu-img create -f qcow2 -o size=2147483648,preallocate=metadata disk.qcow2
Then if you're on Linux, use fallocate to preallocate space:
% fallocate -l 2147483648 disk.qcow2
Passing this on for what it may be worth, they both saved me a lot of time copying files this morning. :)