Improve logging messages in libvirt driver

This is a trivial change to correct an uninformative log message.
There are two checks in get_instance_disk_info that look at a disk.
One checks for the type of a disk and logs its path if it is not a file.
The other checks for the path and logs the instance name it belongs to
if it does not exist. The first will log None if the path does not exist,
so reverse the order of the checks.

Closes-Bug: #1239747
Change-Id: Ieb52ba0584ad789dc55f40e0172cf5ed41e86fbe
Merge "Improve logging messages in libvirt driver"