It was discovered that a race condition existed when rebooting Compute (Nova) nodes. The libvirt-guests script would sometimes start before the Compute service (Nova) itself. As a result both services would attempt to restart virtual machine instances that were running on the host prior to shutdown. The instances would ultimately terminate in error.
PackStack has been updated to ensure that the libvirt-guests script is disabled on Compute nodes, ensuring that the Computer service has full control of restarting virtual machine instances.