conf: Add/Allow parsing the encryption in the disk source

Since the virStorageEncryptionPtr encryption; is a member of
 _virStorageSource it really should be allowed to be a subelement
of the disk <source> for various disk formats:

   Source{File|Dir|Block|Volume}
   SourceProtocol{RBD|ISCSI|NBD|Gluster|Simple|HTTP}

NB: Simple includes sheepdog, ftp, ftps, tftp

That way we can set up to allow the <encryption> element to be
formatted within the disk source, but we still need to be wary
from whence the element was read - see keep track and when it
comes to format the data, ensure it's written in the correct place.

Modify the qemuxml2argvtest to add a parse failure when there is an
<encryption> as a child of <disk> *and* an <encryption> as a child
of <source>.

The virschematest will read the new test files and validate from a
RNG viewpoint things are fine.

docs/formatdomain.html.in

@@ -2712,6 +2712,14 @@
             attribute matching the key that was specified in the
             secret object.
           </dd>
+          <dd><span class="since">Since libvirt 3.9.0</span>, the
+            <code>encryption</code> can be a sub-element of the
+            <code>source</code> element for encrypted storage sources.
+            If present, specifies how the storage source is encrypted
+            See the
+            <a href="formatstorageencryption.html">Storage Encryption</a>
+            page for more information.
+          </dd>
         </dl>
 
         <p>
@@ -3117,8 +3125,11 @@
         <span class="since">Since 0.8.8</span>
       </dd>
       <dt><code>encryption</code></dt>
-      <dd>If present, specifies how the volume is encrypted.  See
-        the <a href="formatstorageencryption.html">Storage Encryption</a> page
+      <dd>Starting with <span class="since">libvirt 3.9.0</span> the
+        <code>encryption</code> element is preferred to be a sub-element
+        of the <code>source</code> element. If present, specifies how the
+        volume is encrypted using "qcow". See the
+        <a href="formatstorageencryption.html">Storage Encryption</a> page
         for more information.
       </dd>
       <dt><code>readonly</code></dt>