customize: Use --no-network flag so test works under UML.

customize/test-virt-customize.sh

@@ -25,7 +25,9 @@ set -e
 for f in ../tests/guests/{debian,fedora,ubuntu}.img; do
     # Ignore zero-sized windows.img if ntfs-3g is not installed.
     if [ -s "$f" ]; then
+        # Add --no-network so UML works.
 	$VG ./virt-customize -n -a $f \
+            --no-network \
             --write /etc/motd:HELLO \
             --delete /etc/motd
     fi