At the moment we allow the user to specify exactly where
they want the HTML documentation to be installed with an
extreme level of precision through the --with-html-dir and
--with-html-subdir configure options.

Most of the time, of course, the user will stick with the
default, that is $(datadir)/doc/$(PACKAGE)-$(VERSION)/html.

So close to $(docdir)! Including the version number in
the path, specifically, seems entirely unnecessary since
different releases of libvirt are not going to be able to
coexist on the same system anyway.

Drop all these custom flexibilty for flexibilty's sake
shenaningans in favor of the standard, well understood
$(docdir).
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Acked-by: NMichal Privoznik <mprivozn@redhat.com>