cpu: Add support for CPU vendor

By specifying <vendor> element in CPU requirements a guest can be
restricted to run only on CPUs by a given vendor. Host CPU vendor is
also specified in capabilities XML.

The vendor is checked when migrating a guest but it's not forced, i.e.,
guests configured without <vendor> element can be freely migrated.

docs/formatcaps.html.in

@@ -22,6 +22,7 @@ BIOS you will see</p>
         &lt;vmx/&gt;
       &lt;/features&gt;
       &lt;model&gt;core2duo&lt;/model&gt;
+      &lt;vendor&gt;Intel&lt;/vendor&gt;
       &lt;topology sockets="1" cores="2" threads="1"/&gt;
       &lt;feature name="lahf_lm"/&gt;
       &lt;feature name='xtpr'/&gt;

docs/formatdomain.html.in

@@ -220,6 +220,7 @@
   ...
   <cpu match='exact'>
     <model>core2duo</model>
+    <vendor>Intel</vendor>
     <topology sockets='1' cores='2' threads='1'/>
     <feature policy='disable' name='lahf_lm'/>
   </cpu>
@@ -267,6 +268,13 @@
         definition can be found in <code>cpu_map.xml</code> file installed
         in libvirt's data directory.</dd>
 
+      <dt><code>vendor</code></dt>
+      <dd><span class="since">Since 0.8.3</span> the content of the
+        <code>vendor</code> element specifies CPU vendor requested by the
+        guest. If this element is missing, the guest can be run on a CPU
+        matching given features regardless on its vendor. The list of
+        supported vendors can be found in <code>cpu_map.xml</code>.</dd>
+
       <dt><code>topology</code></dt>
       <dd>The <code>topology</code> element specifies requested topology of
         virtual CPU provided to the guest. Three non-zero values have to be

docs/schemas/capability.rng

@@ -79,6 +79,11 @@
     <element name='model'>
       <text/>
     </element>
+    <optional>
+      <element name='vendor'>
+        <text/>
+      </element>
+    </optional>
     <element name='topology'>
       <attribute name='sockets'>
         <ref name='positiveInteger'/>

docs/schemas/domain.rng

@@ -1557,6 +1557,7 @@
           <interleave>
             <ref name="cpuModel"/>
             <optional>
+              <ref name="cpuVendor"/>
               <ref name="cpuTopology"/>
             </optional>
             <zeroOrMore>
@@ -1584,6 +1585,12 @@
     </element>
   </define>
 
+  <define name="cpuVendor">
+    <element name="vendor">
+      <text/>
+    </element>
+  </define>
+
   <define name="cpuFeature">
     <element name="feature">
       <attribute name="policy">