Add a note about qcow2 with virt-manager

As noted in the bug report, it is necessary
to perform an additional step when using qcow2
images with virt-manager.

Change-Id: If0cc74fcdaac62d4f2fc5cef89490334cfdd94b8
Closes-Bug: 1258445
Merge "Add a note about qcow2 with virt-manager"