rng: Change the datatype for volume name for common use

The "volName" will be used by later patch.

docs/schemas/basictypes.rng

@@ -225,4 +225,10 @@
     </data>
   </define>
 
+  <define name='volName'>
+    <data type='string'>
+      <param name="pattern">[a-zA-Z0-9_\+\-\.]+</param>
+    </data>
+  </define>
+
 </grammar>

docs/schemas/storagevol.rng

@@ -13,7 +13,7 @@
   <define name='vol'>
     <element name='volume'>
       <element name='name'>
-        <ref name='name'/>
+        <ref name='volName'/>
       </element>
       <optional>
         <element name='key'>
@@ -209,10 +209,4 @@
     </optional>
   </define>
 
-  <define name='name'>
-    <data type='string'>
-      <param name="pattern">[a-zA-Z0-9_\+\-\.]+</param>
-    </data>
-  </define>
-
 </grammar>