docs: Use google sitesearch for website search

The website search is perpetually broken, has had XSS issues in the
past, and I suspect when it's working it's not as fast or capable as
a simple google site:libvirt.org search

Replace the <form> implementation with one that sends the user to
google.com with 'site:libvirt.org' appended to the search string
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>

docs/page.xsl

@@ -155,11 +155,10 @@
             </ul>
           </div>
           <div id="search">
-            <form action="{$href_base}search.php" enctype="application/x-www-form-urlencoded" method="get">
-              <div>
-                <input name="query" type="text" size="12" value=""/>
-                <input name="submit" type="submit" value="Go"/>
-              </div>
+            <form action="https://www.google.com/search" enctype="application/x-www-form-urlencoded" method="get">
+              <input name="sitesearch" type="hidden" value="libvirt.org"/>
+              <input name="q" type="text" size="12" value=""/>
+              <input type="submit" value="Go"/>
             </form>
           </div>
         </div>