Fixes NoneType vcpu list returned by Libvirt driver

* libvirt dom.vcpu may return a NoneType if vcpus dont exist

Change-Id: Iee10c2559ceea636bc4896595962e3470228ad35
Closes-Bug: #1274758
(cherry picked from commit 82821b407ce7a226918758876abca46b727d9706)
Merge "Fixes NoneType vcpu list returned by Libvirt driver" into stable/havana