This has been reported St. Bernard OFM (EVault, Inc.) and they have logged OFM Feature Request 4753. The OFM.NLM product will need to be modified to run in a XEN environment. Please contact them and reference this feature request if you're interested in running OFM.NLM on NetWare running in a XEN virtualized environment.
The STI and CLI instructions should not explicitly be called by an NLM inside a XEN environment. The provided handlers provided by XEN need to be used instead.