conf: Parse and format 'backingStore' for disk <mirror>

When the block copy operation is started with a reused external file in
incremental mode libvirt will need to open and insert the backing chain
for that file into qemu (in -blockdev mode). This means that we'll need
to track the backing chain and metadata such as node names for the full
chain of <mirror>.

This patch invokes the full backing chain formatter and parser for
<mirror> so that the chain can be kept with <mirror>.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

docs/schemas/domaincommon.rng

@@ -5676,6 +5676,7 @@
           </choice>
         </attribute>
       </optional>
+      <ref name="diskBackingChain"/>
     </element>
   </define>
   <define name="diskAuth">