FreeBSD Xen DomU boot from drbd fails with error 19

[niek at bigfoot.com]

Running a FreeBSD 11.0-RELEASE HVM guest, using ufs, on Xen 4.4 (debian
jessie) on LVM with drbd 8.4. The FreeBSD guest boots fine from LVM
using 'disk' /dev/<vg>/<volume> in domU config.

I have a number of Linux guests running on drbd (8.4). Booting FreeBSD
from the drbd device associated with drbd (with 'disk' /dev/drbdx)
fails. I know I can't use drbd:<resource> with HVM guest.

Trying to mount root from ufs:/dev/xbd0p2 [rw]...
mountroot: waiting for device /dev/xbd0p2...
Mounting from ufs:/dev/xbd0p2 failed with error 19.

Device xbd0 is present, but no GPT tables are found.