docs: schema: Add missing <interleave> element to panic device

Panic device has 2 optional sub-elements - <alias> and <address> the
order of which should be interchangeable in the XML.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1456165Signed-off-by: NErik Skultety <eskultet@redhat.com>

docs/schemas/domaincommon.rng

@@ -6070,12 +6070,14 @@
           </choice>
         </attribute>
       </optional>
-      <optional>
-        <ref name="alias"/>
-      </optional>
-      <optional>
-        <ref name="address"/>
-      </optional>
+      <interleave>
+        <optional>
+          <ref name="alias"/>
+        </optional>
+        <optional>
+          <ref name="address"/>
+        </optional>
+      </interleave>
     </element>
   </define>
   <define name="rawIO">