docs: add documentation of arch element of capabilities.xml

Specifically, list sub-elements and where they can be used. In addition,
describe supported machine types for Xen.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>

docs/formatcaps.html.in

@@ -91,7 +91,27 @@
         </dd>
 
         <dt><code>arch</code></dt>
-        <dd>This element brings some information on supported guest architecture.</dd>
+        <dd>This element brings some information on supported guest
+          architecture. Possible subelements are:
+          <dl>
+            <dt><code>wordsize</code></dt><dd>Size of CPU word in bits, for example 64.</dd>
+            <dt><code>emulator</code></dt><dd>Emulator (device model) path, for
+              use in <a href="formatdomain.html#elementEmulator">emulator</a>
+              element of domain XML.</dd>
+            <dt><code>loader</code></dt><dd>Loader path, for use in
+              <a href="formatdomain.html#elementLoader">loader</a> element of domain
+              XML.</dd>
+            <dt><code>machine</code></dt><dd>Machine type, for use in
+              <a href="formatdomain.html#attributeOSTypeMachine">machine</a>
+              attribute of os/type element in domain XML. For example Xen
+              supports <code>xenfv</code> for HVM or <code>xenpv</code> for
+              PV.</dd>
+            <dt><code>domain</code></dt><dd>The <code>type</code> attribute of
+              this element specifies the type of hypervisor required to run the
+              domain. Use in <a href="formatdomain.html#attributeDomainType">type</a>
+              attribute of the domain root element.</dd>
+          </dl>
+        </dd>
 
         <dt><code>features</code></dt>
         <dd>This optional element encases possible features that can be used

docs/formatdomain.html.in

@@ -19,7 +19,8 @@
     <p>
       The root element required for all virtual machines is
       named <code>domain</code>. It has two attributes, the
-      <code>type</code> specifies the hypervisor used for running
+      <a id="attributeDomainType"><code>type</code></a>
+      specifies the hypervisor used for running
       the domain. The allowed values are driver specific, but
       include "xen", "kvm", "qemu", "lxc" and "kqemu". The
       second attribute is <code>id</code> which is a unique
@@ -148,11 +149,11 @@
         (badly named!) refers to an OS that supports the Xen 3 hypervisor
         guest ABI. There are also two optional attributes, <code>arch</code>
         specifying the CPU architecture to virtualization,
-        and <code>machine</code> referring to the machine
+        and <a id="attributeOSTypeMachine"><code>machine</code></a> referring
-        type. The <a href="formatcaps.html">Capabilities XML</a>
+        to the machine type. The <a href="formatcaps.html">Capabilities XML</a>
         provides details on allowed values for
         these. <span class="since">Since 0.0.1</span></dd>
-      <dt><code>loader</code></dt>
+      <dt><a id="elementLoader"><code>loader</code></a></dt>
       <dd>The optional <code>loader</code> tag refers to a firmware blob,
         which is specified by absolute path,
         used to assist the domain creation process. It is used by Xen
@@ -2621,7 +2622,7 @@
 ...</pre>
 
     <dl>
-      <dt><code>emulator</code></dt>
+      <dt><a id="elementEmulator"><code>emulator</code></a></dt>
       <dd>
         The contents of the <code>emulator</code> element specify
         the fully qualified path to the device model emulator binary.