https://bugzilla.redhat.com/show_bug.cgi?id=1526382

Since commit c4eedd79 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: NJohn Ferlan <jferlan@redhat.com>
ACKed-by: NMichal Privoznik <mprivozn@redhat.com>