doc: Add documentation for new cputune elements period and quota

We have added element period and quota. Document them in formatdomain.html.in.

docs/formatdomain.html.in

@@ -307,6 +307,8 @@
     <vcpupin vcpu="2" cpuset="2,3"/>
     <vcpupin vcpu="3" cpuset="0,4"/>
     <shares>2048</shares>
+    <period>1000000</period>
+    <quota>-1</quota>
   </cputune>
   <numatune>
     <memory mode="strict" nodeset="1-4,^3"/>
@@ -400,6 +402,26 @@
         2048 will get twice as much CPU time as a VM configured with value 1024.
         <span class="since">Since 0.9.0</span>
       </dd>
+      <dt><code>period</code></dt>
+      <dd>
+        The optional <code>period</code> element specifies the enforcement
+        interval(unit: microseconds). Within <code>period</code>, each vcpu of
+        the domain will not be allowed to consume more than <code>quota</code>
+        worth of runtime. The value should be in range [1000, 1000000]. A period
+        with value 0 means no value. (NB: Only qemu driver support)
+        <span class="since">Since 0.9.4</span>
+      </dd>
+      <dt><code>quota</code></dt>
+      <dd>
+        The optional <code>quota</code> element specifies the maximum allowed
+        bandwidth(unit: microseconds). A domain with <code>quota</code> as any
+        negative value indicates that the domain has infinite bandwidth, which
+        means that it is not bandwidth controlled. The value should be in range
+        [1000, 18446744073709551] or less than 0. A quota with value 0 means no
+        value. You can use this feature to ensure that all vcpus run at the same
+        speed. (NB: Only qemu driver support)
+        <span class="since">Since 0.9.4</span>
+      </dd>
       <dt><code>numatune</code></dt>
       <dd>
         The optional <code>numatune</code> element provides details of