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().

Introduce a src/libvirt-nwfilter.c file to hold all the
methods related to the virNWFilter type.