At the end I proceeded as follows:
1) I manually installed the package ovirt-node-ng-image-update-placeholder version 4.5.1
on the host; with this change alone, the manager engine began detecting available
updates;
2) before proceeding with the updates, I had to fix my host setup. The error "Local
storage domains were found on the same filesystem as /" was due to the fact that I
was mounting storage at the path /data/normal, while the entire /data path should be on a
different filesystem than root.
After making these changes, upgrade went smoothly to 4.5.3.2
Now I have problems upgrading to 4.5.4, but I think this is a different problem.