Convert a relative file link to a URL in docbook

<xsl:template match="d:link[@role='relative']">
  <xsl:copy>
    <xsl:attribute name="xlink:href">
      <xsl:value-of select="concat('http://prefix/', @xlink:href)"/>
    </xsl:attribute>
  </xsl:copy>
  <xsl:apply-templates select="d:link"/>
</xsl:template>