Only block if when waiting for a non-elapsed time.

Making Xen verify we are setting an event for the future is very much a help.  I do still get odd blocks very rarely (that is, sometimes only N <64k sends/receives complete for BytesStringIVC) and will look into those as time allows.  While the issue might be the same or similar, notice that the remaining buggy behavior is very different - blocking during full IVC operation and not at initial rendezvous.