Fix KeyError for euca-describe-images

EC2 describe images crashes on volume backed instance snapshot which has
several volumes.

Change-Id: Ibe278688b118db01c9c3ae1763954adf19c7ee0d
Closes-bug: #1370265
(cherry picked from commit 1dea1cd710d54d4a2a584590e4ccf59dd3a41faa)