conf: Make backing store index optional

Index will remain an internal property even if we allow backing store
parsing from the XML, so we need to allow backing store without it in
the schema.

docs/schemas/domaincommon.rng

@@ -1415,9 +1415,11 @@
 
   <define name="diskBackingStore">
     <element name="backingStore">
+      <optional>
         <attribute name="index">
           <ref name="positiveInteger"/>
         </attribute>
+      </optional>
       <interleave>
         <ref name="diskSource"/>
         <ref name="diskBackingChain"/>