storage: Disallow create/resize of qcow2 encrypted images
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1526382
    
    Since commit c4eedd793 disallowed qcow2 encrypted images to be
    used for domains, it no longer makes sense to allow a qcow2
    encrypted volume to be created or resized.
    
    Add a test that will exhibit the failure of creation as well
    as the xml2xml validation of the format still being correct.
    
    Update the documentation to note the removal of the capability
    to create and use qcow/default encrypted volumes.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>
    ACKed-by: Michal Privoznik <mprivozn@redhat.com>