Improve debugging and image creation

src/extent.rs

@@ -5,3 +5,12 @@ pub struct Extent {
     pub block: u64,
     pub length: u64,
 }
+
+impl Extent {
+    pub fn new(block: u64, length: u64) -> Extent {
+        Extent {
+            block: block,
+            length: length
+        }
+    }
+}