change the query to check nodes.node_id instead of nodes.phys_nodeid against
the reserved.node_id.

This means the query now returns FBSD-JAIL for vnodes rather than NULL
for the OS.  I ran the query against every current experiment in the DB
and the only other changes were:

  * plab nodes return RHL_PLAB instead of NULL.  As long as linktest is
    not an osfeature of that image (it isn't) then this fix is compatible

  * pc2000s, which host IXPs, now only return one row in the query.
    Previously they would return a row for "pc17X" and for "ixpN".
    This is probably not what we wanted anyway.