Adds instance root disk size checks during resize

Fixes bug: #1163844

The Hyper-V driver does not support disk resizes to a smaller size.
This patch verifies the disk size compatibility before the migration
starts in order to avoid data losses.

Change-Id: Ie99bf8779d583e97b911c9a136cee1bca9a1ecdc
Merge "Adds instance root disk size checks during resize"