Use the description field inputed in the wizard

Make the ImageDescription field of the dialog wizard overwrite the
DESCRIPTION image property.

image_creator/dialog_wizard.py

@@ -187,7 +187,9 @@ def extract_image(session):
 
     #Shrink
     size = device.shrink()
+
     metadata.update(device.meta)
+    metadata['DESCRIPTION'] = wizard['ImageDescription']
 
     #MD5
     md5 = MD5(out)