The "not assigned to domain"-errors appear to be for thin provisioned disks
and for snapshots (which are also shown as thin provisioned by the engine).
One of the errors has gone away once I removed all snapshots from the VM.
Three recurring errors remain:
[root@gnkvm02 ~]# tail -n 1000 /var/log/messages | awk '/not assigned to domain/ {print $8}' | sort | uniq -c
333 vmId=`007ca72e-d0d0-4477-87d4-fb60328cd882`::b526b148-b810-47c6-9bdd-4fd8d8226855/02944008-af6a-4901-b889-1f5fbd02bbd1:
332 vmId=`007ca72e-d0d0-4477-87d4-fb60328cd882`::b526b148-b810-47c6-9bdd-4fd8d8226855/a2da1070-8de7-4e1f-b736-4c88d089a5cc:
335 vmId=`1075a178-a4c6-4a8f-a199-56401cd0652f`::b526b148-b810-47c6-9bdd-4fd8d8226855/341b32d6-4276-454d-b3f0-789b705c99cc:
[root@gnkvm02 ~]#
The first two are for the two thin provisioned disks of a VM with three disks.