Fixes Hyper-V snapshot spawning issue

In Hyper-V, snapshoting an instance and then spawning a new
one using the resulted snapshot image with the same flavor,
will result an Hyper-V Exception.

Hyper-V will get the request to resize the base image to it's
actual size, leading to an error.

The reason is that the logic of the resize_vhd method has been
changed, as it is meant to resize images to the flavor size minus
the image metadata size, thus it's maximum internal size. For this
reason, the way to decide whether the root image should be resized
or not must also be changed.

So, instead of comparing the root image and base image sizes,
we should compare their maximum internal sizes.

Closes-Bug: #1234759

Change-Id: I83d2df76b726d705cedc52a8309a2be7037d6a79
(cherry picked from commit efdca8eeaffe6e63d57243147658f4691da3d89e)

Conflicts:
	nova/tests/test_hypervapi.py
	nova/virt/hyperv/imagecache.py
	nova/virt/hyperv/vmops.py