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.

The methods which get the internal maximum size of a vhd/vhdx
image have been updated to lookup the parent disk and get the according
size instead of raising an exception in case of differencing images.

Change-Id: I867a9b35e5273b96afc443e5c436a15d0c45161b
Implements: blueprint add-differencing-vhdx-resize-support

Conflicts:
	nova/virt/hyperv/vmops.py