Comment 2 for bug 915954

I have found a workaround:

In the /etc/xen/xend-config.sexp file, find and uncommont the lines:
    (xend-unix-server no)
    (xend-unix-path /var/lib/xend/xend-socket)
and change the "(xend-unix-server no)" line to "yes":
    (xend-unix-server yes)

Then reboot.

I request Ubuntu package maintainers add some kind of installation task to the "libvirt" or "virt-manager" package that automatically modifies the "/etc/xen/xend-config.sexp" file so long as the file has not been modified since the time it was installed.