First of all, don't let me wrong: I'm very exited about KVM and libvirt progresses. Now we have not only a very robust hypervisor, but also some critical paravirtualized drivers, a good graphical interface and excellent host / guest remote management capabilities. I would publicly thanks all the talented guys involved in the realization of these great and important projects – thank you boys!
However, it's a shame that the current virt-manager GUI interface don't permit to perform metadata preallocation on QCOW2 image format, as this image is much more feature-rich than the RAW one. Moreover, I would like to see not only the guest creation wizard, but all the guest editing windows to always default to no cache policy for virtual disk, but it is a secondary problem: it is not so difficult to manually change a parameter...
The first problem – no metadata preallocation on QCOW2 – is way more serious, as it can not be overcomed without resort to the command line. This problem should really be corrected as soon as possible. In the meantime, you can use the workaround described above, and remember to always check your virtual disk caching policy – don't use the “default” or “write-through” settings.
I hope than this article can help you to get the max from the very good KVM, libvirt and related projects.