Suppress libarchive logging

libarchive ffi goes crazy logging at *import* time.  Pre-empt
use of the logging with a null handler prior to import.

librarian/imageimporter.py

@@ -1,3 +1,5 @@
+import logging
+logging.getLogger('libarchive').addHandler(logging.NullHandler())
 import libarchive
 import hashlib
 import sys