allowed to allocate a node type. An entry in this table means the type
is restricted; no entries means the type is unrestricted.

Add "isvirtnode" slot to node_types table so that we can put in
entries like pcvron, pcvroninet2, etc. Various parts of the testbed
software will check this flag.

Add phys_nodeid slot to nodes table, which maps a virtnode to its
physical nodes (vron23 maps to ron3). Set phys_nodeid=node_id for
normal testbed nodes.

Change type of priority in the nodes table to a plain int (from
smallint) since I added a bunch of nodes and spread the priority
numbers out pretty far.