blockjob: don't remove older-style mirror XML

Commit 7c6fc39 introduced a regression in the XML produced for older
clients.  The argument at the time was that clients shouldn't be
depending on output-only data for something that is only going to
be triggered for a transient guest; but John Ferlan reported that
the automated testsuite was such a client.  It's better to be safe
than sorry by guaranteeing back-compat cruft.  Note that later
patches will be using <mirror> for active block commit, but there
we don't have to worry about back-compat.

* src/conf/domain_conf.c (virDomainDiskDefFormat): Restore old
style output when necessary.
* docs/schemas/domaincommon.rng: Validate back-compat style.
* docs/formatdomain.html.in: Update the documentation.
* tests/qemuxml2xmloutdata/qemuxml2xmlout-disk-mirror-old.xml:
Update tests.
* tests/qemuxml2argvdata/qemuxml2argv-disk-mirror.xml: Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>

docs/formatdomain.html.in

@@ -1891,11 +1891,13 @@
         attribute <code>ready</code> is present, then it is known the
         disk is ready to pivot; otherwise, the disk is probably still
         copying.  For now, this element only valid in output; it is
-        ignored on input.  <span class="since">Since 1.2.6</span>
-        (Older libvirt <span class="since">since 0.9.12</span> allowed
-        only mirroring to a file, with the information reported
-        in <code>file</code> and <code>format</code> attributes
-        of <code>mirror</code> rather than subelements.)
+        ignored on input.  The <code>source</code> sub-element exists
+        for all two-phase jobs <span class="since">since 1.2.6</span>.
+        Older libvirt supported only block copy to a
+        file, <span class="since">since 0.9.12</span>; for
+        compatibility with older clients, such jobs include redundant
+        information in the attributes <code>file</code>
+        and <code>format</code> in the <code>mirror</code> element.
       </dd>
       <dt><code>target</code></dt>
       <dd>The <code>target</code> element controls the bus / device

docs/schemas/domaincommon.rng

@@ -4176,7 +4176,7 @@
   <define name='diskMirror'>
     <element name='mirror'>
       <choice>
-        <group>
+        <group> <!-- old format, for block copy back-compat -->
           <attribute name='file'>
             <ref name='absFilePath'/>
           </attribute>
@@ -4185,8 +4185,16 @@
               <ref name='storageFormat'/>
             </attribute>
           </optional>
+          <optional>
+            <interleave>
+              <ref name='diskSourceFile'/>
+              <optional>
+                <ref name="diskFormat"/>
+              </optional>
+            </interleave>
+          </optional>
         </group>
-        <group>
+        <group> <!-- preferred format -->
           <interleave>
             <ref name="diskSource"/>
             <optional>