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>

tests/qemuxml2argvdata/qemuxml2argv-disk-mirror.xml

@@ -17,7 +17,7 @@
     <disk type='block' device='disk'>
       <source dev='/dev/HostVG/QEMUGuest1'/>
       <backingStore/>
-      <mirror type='file' ready='yes'>
+      <mirror type='file' file='/dev/HostVG/QEMUGuest1Copy' ready='yes'>
         <source file='/dev/HostVG/QEMUGuest1Copy'/>
       </mirror>
       <target dev='hda' bus='ide'/>
@@ -33,7 +33,7 @@
     <disk type='file' device='disk'>
       <source file='/tmp/data.img'/>
       <backingStore/>
-      <mirror type='file'>
+      <mirror type='file' file='/tmp/copy.img' format='qcow2'>
         <format type='qcow2'/>
         <source file='/tmp/copy.img'/>
       </mirror>