build: Fix API docs generation in VPATH build

XSLT allows for two ways of generating the output of transformation.
Either implicit, which xsltproc prints to stdout and can be redirected
to a file using -o file. Or explicit, which means the stylesheet
contains <xsl:document> element which specifies where the output should
be saved. This can be used for generating more files by a single run of
xsltproc and -o directory/ can change the directory where the output
files will be stored.

devhelp.xsl is special in that it combines both options in one
stylesheet, which doesn't work well with -o:

xsltproc --nonet -o ./devhelp/ ./devhelp/devhelp.xsl ./libvirt-api.xml

Outputs 4 *.html files into ./devhelp but then tries to write to
./devhelp/ as a file (hence the I/O error) rather than writing output to
the fifth file devhelp/libvirt.devhelp.

This patch modifies devhelp.xsl so that all files are generated using
<xsl:document> element and -o directory/ can be used to override output
directory where those files are saved.

docs/devhelp/devhelp.xsl

@@ -13,6 +13,13 @@
   <!-- Build keys for all symbols -->
   <xsl:key name="symbols" match="/api/symbols/*" use="@name"/>
 
+  <xsl:template match="/">
+    <xsl:document xmlns="http://www.devhelp.net/book" href="libvirt.devhelp"
+                  method="xml" encoding="UTF-8" indent="yes">
+      <xsl:apply-templates/>
+    </xsl:document>
+  </xsl:template>
+
   <xsl:template match="/api">
     <book title="{@name} Reference Manual" link="index.html" author="" name="{@name}">
       <xsl:apply-templates select="files"/>