It helps to show your attempt at the solution, then we can tell you where you went wrong. As it is, we can't tell whether you are a novice with no idea where to start, or an expert who overlooked a small point of detail.
It looks to me as if this can be done by
<xsl:template match="li">
<li><xsl:copy-of select="@*|xref"/></li>
<xsl:apply-templates select="li"/>
</xsl:template>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference