Switch from raw images to qcow2 format

This allows to use minimal copy-on-write clones of the base image as
guest images, which in turn saves a lot of disk space.

testing/testing.conf

@@ -40,7 +40,7 @@ LOOPDIR=$BUILDDIR/loop
 ROOTFSSIZE=1024
 ROOTFSSUITE=wheezy
 ROOTFSARCH=amd64
-ROOTFS=$BUILDDIR/debian-$ROOTFSSUITE-$ROOTFSARCH.img
+ROOTFS=$BUILDDIR/debian-$ROOTFSSUITE-$ROOTFSARCH.qcow2
 ROOTFSMIRROR=http://cdn.debian.net/debian
 ROOTFSCOMPILEDIR=$BUILDDIR/compile
 
@@ -48,6 +48,8 @@ ROOTFSCOMPILEDIR=$BUILDDIR/compile
 ROOTFSDIR=$BUILDDIR/root-fs
 
 # libvirt config
+NBDEV=/dev/nbd0
+NBDPARTITION=${NBDEV}p1
 VIRTIMGSTORE=/var/lib/libvirt/images
 
 # Directory where test results will be stored