docs: Fix all links to Xen man pages in html

docs: Fix all links to Xen man pages in html

Second try, this time also works for all links to xen-vbd-interface(7).

We don't try anymore to have pod2html generate relative links, instead
we do it ourself.

First, we modify all links to man pages to have what looks like an
absolute URL and pod2html will just write it in the html output.
Absolute URL in POD are in the form L<text|scheme:...> so let's just use
a scheme that isn't real, but easy to find in the resulting html output:
"relative:".

Then we fix the output and remove all the bogus scheme "relative" and
can end up with nice relative links.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>