Add helper method to determine disk size from instance properties

Add a _disk_size_from_instance method which by using the disk name
determine the size of the disk refered in the instance properties.
Update the method finish_migration to use this new helper.

Change-Id: I45bd3804fdabff35189421d01a0503c886c24a4c
Related-Bug: #1298975
Merge "Add helper method to determine disk size from instance properties"