Add test for get_instance_disk_info to test_virt_drivers

Commit 2bed16c89356554a193a111d268a9587709ed2f7 added
get_instance_disk_info to the base ComputeDriver class
but didn't add a test for it in test_virt_drivers, so
it's added here.

This also fixes the wrong method signature in the fake
virt driver.

Note that this wouldn't have found the problem for the
hyperv or vmware drivers due to bug 1209421.

Change-Id: I5765720d8c3e0a7962b493fa49359514f3136544
Related-Bug: #1358719
Merge "Add test for get_instance_disk_info to test_virt_drivers"