autotest/virt-test — pull request #1897 by will-Do (2014-11-24 09:06:24+00:00)
Title: vol_xml: Catch exception if format may not exist in volume XML

For some types pool(logical, e.g.), volume xml may not contain 'format'
element, so set it to None instead of raise an exception.

Signed-off-by: Yanbing Du