schema: backup: Make 'exportname' and 'exportbitmap' independent

Both are optional but don't have to be specified together. Fix the
schema.

https://bugzilla.redhat.com/show_bug.cgi?id=1826746Suggested-by: NYi Sun <yisun@redhat.com>
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>

docs/schemas/domainbackup.rng

@@ -200,6 +200,8 @@
               <attribute name='exportname'>
                 <text/>
               </attribute>
+            </optional>
+            <optional>
               <attribute name='exportbitmap'>
                 <text/>
               </attribute>