Just following up on this in case anyone else stumbles into this issue.  As
previously mentioned, "the pool was constructed using partition UUIDs," but
I saw neither /dev/disk-uuid or /dev/disk/by-partuuid being constructed on
my "sys" mode installed system, beyond the disk/by-uuid for the system
boot drive.

After poking around for some time, I've got a reasonable workaround.  On a
v3.17 install, editing /etc/grub.d/20_linux_xen, and making the following
modifications:

--- 20_linux_xen.old
+++ 20_linux_xen
@@ -45,7 +45,8 @@

# Default to disabling partition uuid support to maintian compatibility with
# older kernels.
-GRUB_DISABLE_LINUX_PARTUUID=${GRUB_DISABLE_LINUX_PARTUUID-true}
+#GRUB_DISABLE_LINUX_PARTUUID=${GRUB_DISABLE_LINUX_PARTUUID-true}
+GRUB_DISABLE_LINUX_PARTUUID=false

Then running:
$ grub-mkconfig -o /boot/grub/grub.cfg

On reboot I do see the /dev/disk/by-partuuid getting automatically
populated. Since
I don't plan on using a kernel earlier than 3.10 (ref: the GNU GRUB Manual
https://www.gnu.org/software/grub/manual/grub/html_node/Simple-configuration.html),
I feel reasonably safe breaking "compatibility with older kernels."

-- Thom