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.