libdevdctl: Force full match of "timestamp" field name

OpenZFS generates events with a "zio_timestamp" field, which gets mistaken for
"timestamp" by libdevdctl due to imprecise string matching. Then later it is
assumed a "timestamp" field exists when it doesn't and an exception is thrown.

Add a space to the search string so we match exactly "timestamp" rather than
anything with that as a suffix.

Approved by: mav (mentor)
MFC after: 3 days
Sponsored by: iXsystems, Inc.