Adds get_instance_disk_info to compute drivers

Solves a live migration regression issue by adding the
get_instance_disk_info to the compute drivers that do not
have it already implemented.

The method is called by the compute manager.

Change-Id: If5cd9a6f5948c08ef5f1cba2eda31f1175709137
Closes-Bug: #1358719
Co-Authored-By: Ionut Balutoiu <ibalutoiu@cloudbasesolutions.com>
Merge "Adds get_instance_disk_info to compute drivers"