Show host model in domain capabilities

The domain capabilities XML is capable of showing whether each guest CPU
mode is supported or not with a possibility to provide additional
details. This patch enhances host-model capability to advertise the
exact CPU model which will be used as a host-model:

    <cpu>
        ...
        <mode name='host-model' supported='yes'>
            <model fallback='allow'>Broadwell</model>
            <vendor>Intel</vendor>
            <feature policy='disable' name='aes'/>
            <feature policy='require' name='vmx'/>
        </mode>
        ...
    </cpu>

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>

docs/formatdomaincaps.html.in

@@ -154,7 +154,12 @@
   ...
   <cpu>
     <mode name='host-passthrough' supported='yes'/>
-    <mode name='host-model' supported='yes'/>
+    <mode name='host-model' supported='yes'>
+      <model fallback='allow'>Broadwell</model>
+      <vendor>Intel</vendor>
+      <feature policy='disable' name='aes'/>
+      <feature policy='require' name='vmx'/>
+    </mode>
     <mode name='custom' supported='yes'>
       <model usable='no'>Broadwell</model>
       <model usable='yes'>Broadwell-noTSX</model>
@@ -177,7 +182,19 @@
       <dd>No mode specific details are provided.</dd>
 
       <dt><code>host-model</code></dt>
-      <dd>No mode specific details are provided yet.</dd>
+      <dd>
+        If <code>host-model</code> is supported by the hypervisor, the
+        <code>mode</code> describes the guest CPU which will be used when
+        starting a domain with <code>host-model</code> CPU. The hypervisor
+        specifics (such as unsupported CPU models or features, machine type,
+        etc.) may be accounted for in this guest CPU specification and thus
+        the CPU can be different from the one shown in host capabilities XML.
+        This is indicated by the <code>fallback</code> attribute of the
+        <code>model</code> sub element: <code>allow</code> means not all
+        specifics were accounted for and thus the CPU a guest will see may
+        be different; <code>forbid</code> indicates that the CPU a guest will
+        see should match this CPU definition.
+      </dd>
 
       <dt><code>custom</code></dt>
       <dd>

docs/schemas/domaincaps.rng

@@ -2,6 +2,7 @@
 <!-- A Relax NG schema for the libvirt domain capabilities XML format -->
 <grammar xmlns="http://relaxng.org/ns/structure/1.0" datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
   <include href='basictypes.rng'/>
+  <include href='cputypes.rng'/>
   <start>
     <ref name='domainCapabilities'/>
   </start>
@@ -94,6 +95,15 @@
         <value>host-model</value>
       </attribute>
       <ref name='supported'/>
+      <optional>
+        <ref name="cpuModel"/>
+        <optional>
+          <ref name="cpuVendor"/>
+        </optional>
+        <zeroOrMore>
+          <ref name="cpuFeature"/>
+        </zeroOrMore>
+      </optional>
     </element>
   </define>