Fix XML search for "privilege" elements in credentials.
Fix XML search for "privilege" elements in credentials.

The previous code searched for _all_ privileges, which returned
extraneous results in case of delegation.  We are now more precise
about the locations in the tree we're considering.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome