Move to a more canonicalized output from qemu-img info.

Move to a form that is all lower cased, dashes->underscores, underscores
instead of spaces which allows for better integration with python. Also
make the parser more robust to failures when encountering new fields such
as snapshot lists. Provide a new qemu img info object that can be used
to do the parsing and access the underlying attributes.

Change-Id: Ie098dbd9f06dd4ef966768e2caa128f1d09b019c