I posted a report on the xen mailing list and the devs are narrowing it down to missing CPU info in the ACPI tables.
mkdir acpi-tables
cd acpi-tables
cp /sys/firmware/acpi/tables/* .
iasl -d *
grep -r -e _PCT -e _PPC -e _PSS *.dsl
This de-compiles the ACPI tables and checks for CPU info. In my case, the grep command returns nothing. This information is what Xen normally needs dom0 to upload to it.
Do you also have a librebooted T480S? If you or anyone else does, it would be very helpful if you could please run the above commands and report the results here.
For anyone interested, the thread on xen-devel mailing list is here and continued here.