vol_xml: Catch exception if format 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 <ydu@redhat.com>
Merge pull request #1897 from will-Do/vol_info_fix

vol_xml: Catch exception if format not exist in volume XML