Here are two munin plugins that monitor disk and network I/O of virtual machines using libvirt. These are also two simple examples on using the libvirt python bindings. You need python-libvirt and python-libxml2 installed in order to use them. If you don't want to use the default uri, try:
[libvirt-*] env.uri qemu:///system
Munin-node needs to have read access to libvirt's socket. This is the case in Debian's default installation.