Adds call to mknod if loop dev does not exist after call to losetup -f (#483)

export-image/prerun.sh

@@ -35,7 +35,7 @@ if [ "${NO_PRERUN_QCOW2}" = "0" ]; then
 
 	echo "Creating loop device..."
 	cnt=0
-	until LOOP_DEV="$(losetup --show --find --partscan "$IMG_FILE")"; do
+	until ensure_next_loopdev && LOOP_DEV="$(losetup --show --find --partscan "$IMG_FILE")"; do
 		if [ $cnt -lt 5 ]; then
 			cnt=$((cnt + 1))
 			echo "Error in losetup.  Retrying..."