Add differencing vhdx resize support in Hyper-V Driver

Differencing VHDX images can be resized, unlike differencing
VHD images. Even so, the Nova Hyper-V driver currently does
not support this.

This feature is required for resizing existing instances which
use CoW VHDX images and also in order to resize the root disk
image when spawning a new instance.

Change-Id: Iba8ea427776ed1fc90e3b326662a1ddf07898cd1
Merge "Add differencing vhdx resize support in Hyper-V Driver"