In Ubuntu we've changed libvirt to connect to qemu:///session rather
than xen:/// if no URI is passed to virConnectOpen. fence_xvmd
incidentally does not pass a URI to virConnectOpen and hence will fail.
This patch fixes this old and deprecated use of virConnectOpen.
(Also available as http://people.ubuntu.com/~soren/libvirt_xen_uri.diff)