Added timestamps to storage volumes

The access, birth, modification and change times are added to
storage volumes and corresponding xml representations.  This
shows up in the XML in this format:

<timestamps>
  <atime>1341933637.027319099</atime>
  <mtime>1341933637.027319099</mtime>
</timestamps>

Signed-off-by: Eric Blake <eblake@redhat.com>

docs/formatstorage.html.in

@@ -141,6 +141,11 @@
             <mode>0744</mode>
             <label>virt_image_t</label>
           </permissions>
+          <timestamps>
+            <atime>1341933637.273190990</atime>
+            <mtime>1341930622.047245868</mtime>
+            <ctime>1341930622.047245868</ctime>
+          </timestamps>
           <encryption type='...'>
             ...
           </encryption>
@@ -172,6 +177,19 @@
         contains the MAC (eg SELinux) label string.
         <span class="since">Since 0.4.1</span>
       </dd>
+      <dt><code>timestamps</code></dt>
+      <dd>Provides timing information about the volume. Up to four
+        sub-elements are present,
+        where <code>atime</code>, <code>btime</code>, <code>ctime</code>
+        and <code>mtime</code> hold the access, birth, change and
+        modification time of the volume, where known. The used time
+        format is &lt;seconds&gt;.&lt;nanoseconds&gt; since the
+        beginning of the epoch (1 Jan 1970). If nanosecond resolution
+        is 0 or otherwise unsupported by the host OS or filesystem,
+        then the nanoseconds part is omitted.  This is a readonly
+        attribute and is ignored when creating a volume.
+        <span class="since">Since 0.10.0</span>
+      </dd>
       <dt><code>encryption</code></dt>
       <dd>If present, specifies how the volume is encrypted.  See
         the <a href="formatstorageencryption.html">Storage Encryption</a> page