Fixes vhdx cow image resizing issue

Fixes Hyper-V differencing image resizing

VHDX differencing images cannot be resized on Hyper-V as the
requested size is not specified when creating the image.

When creating an instance using VHDX differencing images, the base
image is not resized like in the case of VHD. Instead, the flavor
size must be specified when creating the root image.

The method which gets the internal size of an image for a
requested maximum phyisical size raises an exception in case of
differencing images. Instead, this method should be called on
the parent image, getting the requested size.

Fixes bug: #1294682

Change-Id: I460f4e27907b1b10489019266d7eb9bb26292261

Conflicts:
	nova/tests/virt/hyperv/test_vhdutils.py
	nova/virt/hyperv/vmops.py