Ah, it appears that the limit is 2TB because the VMs are created with a 
'msdos' partition table on the virtual disks.  I think we need them to 
be 'gpt'....  parted shows the disks are big, just the filesystems are 
small:
[centos@bigtest2 ~]$ sudo parted /dev/vda
GNU Parted 3.1
Using /dev/vda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) p
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 10.7TB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number  Start   End     Size    Type     File system  Flags
  1      1049kB  1941GB  1941GB  primary  xfs          boot
Still digging for info on how to use gpt with OpenStack/libvirt.... 
Nothing yet...
On 11/20/2015 02:51 PM, Erich Weiler wrote:
 Hi Y'all,
 I have a bunch of OpenStack Nova nodes, each with 11TB of local disk for
 Cloud VMs.  This is all on RDO - RHEL 7 on OpenStack Kilo.  That local
 storage is configured with a XFS filesystem.
 If I specify 8TB on 'root filesystem size' in a flavor, it only seems to
 be able to make a 2TB root filesystem on the guest.  No matter what
 guest OS I use (I've tried CentOS 7.1, Ubuntu 15.10, others) it only
 seems to be able to create a 2TB root filesystem.  I've also tried
 making a 5GB root filesystem and specifying 8TB for ephemeral disk, but
 the ephemeral disk that shows up on /tmp in the guest is only 2TB as well.
 Is there a 2TB KVM limit on VM size?  Max file size in ext4 and XFS is
 well beyond 2TB, so I don't think that's it...  Maybe a limit in KVM or
 libvirt?
 The qemu xml file on the node does specify a 8TB disk....
 Any insight most welcome!!
 cheers,
 erich