util: conf: Don't log when adding commented out lines
util: conf: Don't log when adding commented out lines

virConfAddEntry spams debug logs even for fully commented out lines.
Skip such messages to avoid:

2017-05-12 12:35:38.867+0000: 10820: debug : virConfAddEntry:241 : Add entry (null) (nil)
2017-05-12 12:35:38.867+0000: 10820: debug : virConfAddEntry:241 : Add entry (null) (nil)
2017-05-12 12:35:38.867+0000: 10820: debug : virConfAddEntry:241 : Add entry (null) (nil)
2017-05-12 12:35:38.867+0000: 10820: debug : virConfAddEntry:241 : Add entry (null) (nil)
2017-05-12 12:35:38.867+0000: 10820: debug : virConfAddEntry:241 : Add entry (null) (nil)
...

This also fixes NULL passed to printf.

olafhering/libvirt,datto/libvirt,libvirt/libvirt,libvirt/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,olafhering/libvirt,olafhering/libvirt,crobinso/libvirt,eskultety/libvirt,nertpinx/libvirt,jardasgit/libvirt,andreabolognani/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,datto/libvirt,zippy2/libvirt,crobinso/libvirt,eskultety/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,jardasgit/libvirt,jfehlig/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,zippy2/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,zippy2/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,jfehlig/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,crobinso/libvirt,jardasgit/libvirt,andreabolognani/libvirt,libvirt/libvirt,VenkatDatta/libvirt,datto/libvirt,datto/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,jfehlig/libvirt,olafhering/libvirt,eskultety/libvirt,zippy2/libvirt,crobinso/libvirt,datto/libvirt,eskultety/libvirt,eskultety/libvirt