Comment 1 for bug 1367220

Observed the following in /var/log/libvirtd.log when this occurs:
root@nodec3:~# tail -f /var/log/libvirt/libvirtd.log &
root@nodec3:/usr/lib/python2.7/dist-packages/nova# nova interface-attach --port-id c9dc8241-a9e3-4f48-9829-6176e20e0a63 vm1
2014-10-20 22:46:19.357+0000: 1679: error : qemuMonitorJSONCheckError:354 : internal error: unable to execute QEMU command 'getfd': No file descriptor supplied via SCM_RIGHTS
ERROR: Failed to attach interface (HTTP 500) (Request-ID: req-ed274411-2e79-4eca-9ee6-0e43ba197eff)

Traced this to:
https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1322568

Fixed in a version of libvirt available for trusty but not for precise.