debug: assure NULLSTR() around all %s args in debug at top of public APIs

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().