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>
(cherry picked from commit 9c9e44e111abafde1e05a5e38f7e1645ec8ba6df)
Removes unnecessary instructions in test_hypervapi

Lines have no actual effect on the tests. Removing them
will facilitate further unit tests.

Partial-Bug: #1220256

Change-Id: I805aba5e75f2640221b2dc0034950c0832841247
(cherry picked from commit 0d9bab118feb587d3ac7859b8cf17b8b1388bf2c)
Merge "Adds get_instance_disk_info to compute drivers" into stable/icehouse
Merge "Removes unnecessary instructions in test_hypervapi" into stable/icehouse
Merge remote-tracking branch 'upstream/stable/icehouse' into stable/icehouse