Use xz to compress ubuntu-image .img files before transfer, so that we

have smaller downloads and retain file sparseness everywhere.

debian/changelog

@@ -2,6 +2,8 @@ livecd-rootfs (2.443) UNRELEASED; urgency=medium
 
   * Mark autopkgtests isolation-machine since debootstrap won't work in a
     container.
+  * Use xz to compress ubuntu-image .img files before transfer, so that we
+    have smaller downloads and retain file sparseness everywhere.
 
  -- Steve Langasek <steve.langasek@ubuntu.com>  Fri, 28 Apr 2017 17:41:05 -0700
 

live-build/auto/build

@@ -17,7 +17,10 @@ fi
 
 if [ "${IMAGEFORMAT:-}" = "ubuntu-image" ]; then
 	# Use ubuntu-image instead of live-build
+	apt-get install -qqy pxz
+
 	env SNAPPY_STORE_NO_CDN=1 ubuntu-image -o livecd."$PROJECT".img livecd."$PROJECT".model-assertion
+	pxz -T4 livecd."$PROJECT".img
 	exit 0
 fi