schema: Move timer element inside clock

Both conf/domain_conf.c and domain XML documentation have <timer>
elements inside /domain/clock. Change domain.rng schema to be consistent
with them.

docs/schemas/domain.rng

@@ -354,9 +354,6 @@
     </interleave>
   </define>
   <define name="clock">
-    <zeroOrMore>
-      <ref name="timer"/>
-    </zeroOrMore>
     <optional>
       <element name="clock">
         <choice>
@@ -387,7 +384,9 @@
             </optional>
           </group>
         </choice>
-        <empty/>
+        <zeroOrMore>
+          <ref name="timer"/>
+        </zeroOrMore>
       </element>
     </optional>
   </define>

tests/domainschemadata/timers.xml

+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory>219200</memory>
+  <currentMemory>219200</currentMemory>
+  <vcpu>1</vcpu>
+  <os>
+    <type arch='i686' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <clock offset='utc'>
+    <timer name='pit' tickpolicy='delay'/>
+    <timer name='hpet'/>
+  </clock>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <disk type='block' device='disk'>
+      <source dev='/dev/HostVG/QEMUGuest1'/>
+      <target dev='hda' bus='ide'/>
+      <address type='drive' controller='0' bus='0' unit='0'/>
+    </disk>
+    <controller type='ide' index='0'/>
+    <memballoon model='virtio'/>
+  </devices>
+</domain>