[libvirt] PATCH: add support for -snapshot

Hi, i'm working on some infrastructure to run unit tests in a virtual environment. For this, I need to be able to pass a "snapshot=on" argument to a qemu virtual disk so that I know I start with the same state all the time. The attached patch implements this. It adds a boolean <snapshot/> to the <disk> section in the XML. Please consider for inclusion. Regards, Geert Jansen