docs: fix 404 page when fetched from different location

Error 404 page had relative paths specified for both the image and
stylesheets which caused a problem when requested URL included a
subfolder (e.g. http://libvirt.org/asdf/asdf ). This patch corrects
this behaviour by modifying href_base to '/' (for style-sheets) and
changing the src of the image (to be '/' always).

docs/404.html.in

@@ -15,7 +15,7 @@
     </ul>
 
     <p class="image">
-      <img src="libvirtLogo404.png" alt="libvirt Logo"/>
+      <img src="/libvirtLogo404.png" alt="libvirt Logo"/>
     </p>
 
 

docs/site.xsl

@@ -14,7 +14,16 @@
     doctype-public="-//W3C//DTD XHTML 1.0 Strict//EN"
     doctype-system="http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"/>
 
-  <xsl:variable name="href_base" select="''"/>
+  <xsl:variable name="href_base">
+    <xsl:choose>
+      <xsl:when test="$pagename = '404.html'">
+        <xsl:value-of select="'/'"/>
+      </xsl:when>
+      <xsl:otherwise>
+        <xsl:value-of select="''"/>
+      </xsl:otherwise>
+    </xsl:choose>
+  </xsl:variable>
 
   <xsl:template match="/">
     <xsl:apply-templates select="." mode="page">