Fixes Hyper-V VHDX snapshot bigger than instance

In Hyper-V, the deployed images will be extended having their
metadata size added to their base image size. This may lead to
snapshots bigger than the flavor size.

This patch adds support to the V2 utils module, so that the
maximum internal size of a requested image may be calculated
in a similar way it is done in the V1 module.

VHDX format specs:
http://www.microsoft.com/en-us/download/details.aspx?id=34750

Closes-bug:  #1231911

Change-Id: I0d6aacb275f8af24554d3e8b6f04fe8a454b913f