Use @property to create the os attribute in Image

Use the @property decorator instead of the property() method to create a
read-only os attribute in Image class.

image_creator/image.py

@@ -180,7 +180,8 @@ class Image(object):
         self.guestfs_enabled = False
         self.out.success('done')
 
-    def _get_os(self):
+    @property
+    def os(self):
         """Return an OS class instance for this image"""
         if hasattr(self, "_os"):
             return self._os
@@ -195,8 +196,6 @@ class Image(object):
 
         return self._os
 
-    os = property(_get_os)
-
     def destroy(self):
         """Destroy this Image instance."""