xenapi: boot from volume is broken

Using exercises/boot_from_volume.sh, the test fails at:
+ local exitcode=0
+ set +o xtrace
+ timeout 500 sh -c 'while ! nova show 92ddbd18-5725-4c3b-80a4-8cbcbecadb3d | grep status | grep -q ACTIVE; do sleep 1; done'

At n-cpu, you see this:

Traceback (most recent call last):
  File "/opt/stack/nova/nova/compute/manager.py", line 747, in _spawn
    current_power_state = self._get_power_state(context, instance)
  File "/opt/stack/nova/nova/virt/xenapi/driver.py", line 179, in spawn
    admin_password, network_info, block_device_info)
  File "/opt/stack/nova/nova/virt/xenapi/vmops.py", line 381, in spawn
    undo_mgr.rollback_and_reraise(msg=msg, instance=instance)
  File "/opt/stack/nova/nova/utils.py", line 1315, in rollback_and_reraise
    self._rollback()
  File "/usr/lib/python2.7/contextlib.py", line 24, in __exit__
    self.gen.next()
  File "/opt/stack/nova/nova/virt/xenapi/vmops.py", line 370, in spawn
    vm_ref = create_vm_step(undo_mgr, vdis, kernel_file, ramdisk_file)
  File "/opt/stack/nova/nova/virt/xenapi/vmops.py", line 135, in inner
    rv = f(*args, **kwargs)
  File "/opt/stack/nova/nova/virt/xenapi/vmops.py", line 311, in create_vm_step
    rescue=rescue)
  File "/opt/stack/nova/nova/virt/xenapi/vmops.py", line 404, in _create_vm
    vdis['root']['ref'], disk_image_type, instance['os_type'])
KeyError: 'root'