Adds Hyper-V VHDX support

Blueprint: hyper-v-vhdx

The VHDX virtual disk format has been introduced with Hyper-V Server 2012
providing performance and management improvements.

Unlike the VHD format, VHDX allows to resize differencing disks. This
reduces the requirements for local disk space in case of CoW images and
improves the performance of the first spawn operation for any given image
and flavor combination.

Supporting the VHDX format in Hyper-V requires the WMI V2 namespace.

Unit tests have been added accordingly.

Change-Id: Idddec06b9639e22d69426562d1b5a0cbf2890ccf
Merge "Adds Hyper-V VHDX support"