Add support for setting disk drive serial numbers

* docs/schemas/domain.rng: Add <serial> element to disks
* src/domain_conf.h, src/domain_conf.c: XML parsing and
  formatting for disk serial numbers
* src/qemu_conf.c: Set serial number when launching guests
* tests/qemuxml2argvdata/qemuxml2argv-disk-drive-shared.args,
  tests/qemuxml2argvdata/qemuxml2argv-disk-drive-shared.xml: Add
  serial number to XML test

docs/schemas/domain.rng

@@ -347,6 +347,11 @@
         <empty/>
       </element>
     </optional>
+    <optional>
+      <element name="serial">
+        <ref name="diskSerial"/>
+      </element>
+    </optional>
     <optional>
       <ref name="encryption"/>
     </optional>
@@ -1169,6 +1174,11 @@
       <param name="pattern">[A-Za-z0-9_\.\+\-&amp;:/]+</param>
     </data>
   </define>
+  <define name="diskSerial">
+    <data type="string">
+      <param name="pattern">[A-Za-z0-9_\.\+\-]+</param>
+    </data>
+  </define>
   <define name="genericName">
     <data type="string">
       <param name="pattern">[a-zA-Z0-9_\+\-]+</param>