Allow multiple panic devices

'model' attribute was added to a panic device but only one panic
device is allowed. This patch changes panic device presence
from 'optional' to 'zeroOrMore'.

docs/schemas/domaincommon.rng

@@ -4044,9 +4044,9 @@
         <optional>
           <ref name="nvram"/>
         </optional>
-        <optional>
+        <zeroOrMore>
           <ref name="panic"/>
-        </optional>
+        </zeroOrMore>
       </interleave>
     </element>
   </define>