DescribeInstances in ec2 shows wrong image-message

If one instance boots from a volume,
the value of 'image_id' by 'DescribeInstances' in ec2,
will be a wrong number.

The value of 'image_ref' for this type of instances, is '', not None.
So the original protected codes can't take efforts.

Change-Id: I8c5e1c626f2b7873e7720f3615db1ee1ec6c681a
Closes-Bug: #1278696
Merge "DescribeInstances in ec2 shows wrong image-message"