Allow a timezone to be specified instead of sync to host timezone

This extends the XML to allow for

  <clock offset='timezone' timezone='Europe/Paris'/>

This is useful if the admin has not configured any timezone on the
host OS, but still wants to synchronize a guest to a specific one.

* src/conf/domain_conf.h, src/conf/domain_conf.c: Support extra
  'timezone' attribute on clock configuration
* docs/schemas/domain.rng: Add 'timezone' attribute
* src/xen/xend_internal.c, src/xen/xm_internal.c: Reject configs
  with a configurable timezone

docs/schemas/domain.rng

@@ -304,6 +304,16 @@
           <attribute name="offset">
             <value>utc</value>
           </attribute>
+          <group>
+            <attribute name="offset">
+              <value>timezone</value>
+            </attribute>
+            <optional>
+              <attribute name="timezone">
+                <ref name="timeZone"/>
+              </attribute>
+            </optional>
+          </group>
           <group>
             <attribute name="offset">
               <value>variable</value>
@@ -1633,4 +1643,9 @@
       <param name="pattern">(-|\+)?[0-9]+</param>
     </data>
   </define>
+  <define name="timeZone">
+    <data type="string">
+      <param name="pattern">[a-zA-Z0-9_\.\+\-/]+</param>
+    </data>
+  </define>
 </grammar>