debug: assure NULLSTR() around all %s args in debug at top of public APIs
    
    There are also a couple that were very uninformatively just logging
    the value of the pointer rather than the string itself:
    
    * the "name" arg to virNodeDeviceLookupByName()
    * wwnn and wwpn args to virNodeDeviceLookupSCSIHostByWWN()
    
    All char*'s that make sense should now have their contents logged
    rather than the pointer, and all %s args should now be inside
    NULLSTR().