Expose a host UUID in the capabilities XML

Allow for a host UUID in the capabilities XML. Local drivers
will initialize this from the SMBIOS data. If a sanity check
shows SMBIOS uuid is invalid, allow an override from the
libvirtd.conf configuration file

* daemon/libvirtd.c, daemon/libvirtd.conf: Support a host_uuid
  configuration option
* docs/schemas/capability.rng: Add optional host uuid field
* src/conf/capabilities.c, src/conf/capabilities.h: Include
  host UUID in XML
* src/libvirt_private.syms: Export new uuid.h functions
* src/lxc/lxc_conf.c, src/qemu/qemu_driver.c,
  src/uml/uml_conf.c: Set host UUID in capabilities
* src/util/uuid.c, src/util/uuid.h: Support for host UUIDs
* src/node_device/node_device_udev.c: Use the host UUID functions
* tests/confdata/libvirtd.conf, tests/confdata/libvirtd.out: Add
  new host_uuid config option to test

docs/schemas/capability.rng

@@ -18,6 +18,11 @@
 
   <define name='hostcaps'>
     <element name='host'>
+      <optional>
+        <element name='uuid'>
+          <ref name='UUID'/>
+        </element>
+      </optional>
       <element name='cpu'>
         <element name='arch'>
           <ref name='archnames'/>
@@ -349,4 +354,15 @@
       <param name='pattern'>[a-zA-Z0-9\-_]+</param>
     </data>
   </define>
+
+  <define name="UUID">
+    <choice>
+      <data type="string">
+        <param name="pattern">[a-fA-F0-9]{32}</param>
+      </data>
+      <data type="string">
+        <param name="pattern">[a-fA-F0-9]{8}\-([a-fA-F0-9]{4}\-){3}[a-fA-F0-9]{12}</param>
+      </data>
+    </choice>
+  </define>
 </grammar>