Comment 31 for bug 1446177

I have to go back to the libvirt code tomorrow. If I remember correctly it was using the physical_id info (which is also seen in /proc/cpuinfo) as the number of sockets. At least in the 14.04 version of libvirt. Might be something to explicitly check in newer versions. Since you already tested a recent kernel we can at least rule out that side. So it is very likely a change in libvirt.
Unfortunately I think it will not be as straight forward to try a newer libvirt as that likely has more dependencies. When I look into more details tomorrow, I try to build a 15.04 version of libvirt in a 15.04 build environment. The result should go only in a test environment as it will never get updated (just a cautious warning).