addition to to being free, the underlying physnode has to either be
free, or not in the hwdown group. Either way, the the underlying
physnode has to be "up", as determined by the autostatus stuff; this
will prevent us from allocating a dead virtual node to an experiment.
The hwdown stuff is especially hacky; we need a better way of
determining what nodes are available to be multiplxed. This will do for
the (very) short term.