Comment 16 for bug 550409

As already mentioned, virt-manager 0.8.5+ supports setting the qemu cache mode for virtual disks. I've posted instructions on my blog on how to compile this virt-manager with the latest virtinst in an arbitrary directory (ie. without installing it globally in /usr) for testing. It works just fine for me with my Lucid based host OS. For compiling virt-manager and virtinst I used a Karmic desktop, but it's most probably not that important (it should work on Jaunty, Karmic, Lucid, ...).
http://muzso.hu/node/4675