We'll contain each slide in a [fo:]block:

<xsl:template match="s:slides/s:slide">
<fo:block>
  <xsl:if test="following-sibling::s:slide">
    <xsl:attribute name="break-after">page</xsl:attribute>
  </xsl:if>
  <xsl:apply-templates/>
  </fo:block>
</xsl:template>

[fo:]block can contain other [fo:]block much like XHTML's div can contain p elements.

The break-after attribute tells the page to break after the slide.

We only want a page break between slides.