Use xz -T4 -0 instead of pxz

Test showed very little difference in file-size between -0 and -9 while
both compression and decompression need much less memory and compression
needs less CPU time for -0.

Supported xz versions before 5.2 also accept the -T4 option but ignore it.

Also depend on xz-utils instead of downloading pxz.

LP: #1701132

debian/control

@@ -31,6 +31,7 @@ Depends: ${misc:Depends},
          rsync,
          squashfs-tools (>= 1:3.3-1),
          vmdk-stream-converter [amd64 i386],
+         xz-utils,
          zerofree
 Suggests: partimage
 Breaks: ubuntu-defaults-builder (<< 0.32)

live-build/ubuntu-cpc/hooks/040-qcow2-image.binary

@@ -4,8 +4,7 @@ case $ARCH:$SUBARCH in
 	# Not sure if any other cloud images use subarch for something that
 	# should take qcow2 format, so only skipping this on raspi2 for now.
 	armhf:raspi2)
-		apt-get install -qqy pxz
-		pxz -T4 -c binary/boot/disk.ext4 > livecd.ubuntu-cpc.disk1.img.xz
+		xz -0 -T4 -c binary/boot/disk.ext4 > livecd.ubuntu-cpc.disk1.img.xz
 	        exit 0
 		;;
 esac