postConfigure uses invalid device nodes for dbroots

During postConfigure for a new cluster, we are choosing external storage for our Performance Modules in order to enable HA failover reattachment of EBS volumes to alternate PMs in the event of a PM failure.

Whether one chooses to allow postConfigure to create new EBS volumes for the PM dbroots or create the volumes their self and provide the volume IDs, postConfigure incorrectly assumes the devfs node for the dbroot to be a legacy Xen device descriptor (such as /dev/xvdg, /dev/xvdh, etc.) and writes these incorrect values to /etc/fstab.