rng: Only one secret is in vol instead of zero or more

Referring to commit fab2e49d, it should be one and only secret for encryption.
Signed-off-by: NHan Han <hhan@redhat.com>

docs/schemas/storagecommon.rng

@@ -24,9 +24,7 @@
         </choice>
       </attribute>
       <interleave>
-        <zeroOrMore>
-          <ref name='secret'/>
-        </zeroOrMore>
+        <ref name='secret'/>
         <optional>
           <element name='cipher'>
             <ref name='keycipher'/>