Add a note about qcow2 with virt-manager
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

saeki-masaki/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,kairoaraujo/openstack-manuals,potsmaster/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,Akasurde/openstack-manuals,Akasurde/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,gongysh2004/openstack-manual,gongysh2004/openstack-manual,potsmaster/openstack-manuals,saeki-masaki/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals