Replace references to old wiki with ones to new.

Replace references to old wiki with ones to new.

I have confirmed that the relevant pages have been transitioned.

What remains is pages which have not yet been moved over:

$ rgrep xenwiki *
tools/libxen/README:http://wiki.xensource.com/xenwiki/XenApi
tools/xenballoon/xenballoond.README:http://wiki.xensource.com/xenwiki/Open_Topics_For_Discussion?action=AttachFile&do=get&target=Memory+Overcommit.pdf

Note that "PythonInXlConfig" never existed in the old wiki and does not exist
in the new. This reference was introduced by 22735:cb94dbe20f97 and was
supposed to have been written prior to the 4.1 release. I have transitioned it
anyway but it's not clear how valuable the message actually is. Perhaps we
should just remove that aspect of it?

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>