docs: Correct invalid hyperlinks
    
    Since libvirt.h was split into multiple files and similarly
    docs/libvirt-libvirt.html, docs/hvsupport.html have bad hyperlinks.  The
    same happens for all the html.in files that used <code class='docref'>
    tag, because page.xsl has no idea where to point the link that's found.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>