docs: don't run xmllint when generating API HTML files

After generating the API HTML files we run xmllint in docs/html/*.html
to validate the correctness. Since

  commit 0aa8536f
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Nov 20 14:49:26 2019 +0000

    docs: generate API reference pages for admin, qemu & lxc libraries

we have many rules generating files into docs/html/. The xmllint
calls for each rule are picking up files which are part-generated by
other parallel build rules resulting in transient errors like:

  GEN      html/index.html
  GEN      html/index-admin.html
  GEN      html/index-qemu.html
  GEN      html/index-lxc.html
  GEN      hvsupport.html.in
html/index-lxc.html:1: parser error : Document is empty

^
make[4]: *** [Makefile:2407: html/index-qemu.html] Error 1

The easiest solution is to move the xmllint rules to the 'make check'
phase of the build.
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

docs/Makefile.am

@@ -311,16 +311,19 @@ html/index.html: libvirt-api.xml newapi.xsl page.xsl $(APIBUILD_STAMP)
 	$(AM_V_GEN)$(XSLTPROC) --nonet -o ./ \
 	  --stringparam builddir '$(abs_top_builddir)' \
 	  --stringparam timestamp $(timestamp) \
-	  $(srcdir)/newapi.xsl libvirt-api.xml && \
-	  $(XMLLINT) --nonet --noout html/*.html
+	  $(srcdir)/newapi.xsl libvirt-api.xml
 
 html/index-%.html: libvirt-%-api.xml newapi.xsl page.xsl $(APIBUILD_STAMP)
 	$(AM_V_GEN)$(XSLTPROC) --nonet -o ./ \
 	  --stringparam builddir '$(abs_top_builddir)' \
 	  --stringparam timestamp $(timestamp) \
 	  --stringparam indexfile $(@:html/%=%) \
-	  $(srcdir)/newapi.xsl $< && \
-	  $(XMLLINT) --nonet --noout html/*.html
+	  $(srcdir)/newapi.xsl $<
+
+check-html:
+	$(XMLLINT) --nonet --noout html/*.html
+
+check-local: check-html
 
 python_generated_files = \
 		html/libvirt-libvirt-lxc.html \