README: correct links (should be xenproject.org, not xen.org)

Signed-off-by: David Scott <dave.scott@citrix.com>
README: don't use "XCP" any more; don't use opam; point people at xenserver-core

The XCP story is mostly historical and quite confusing.
The opam build instructions don't work but the xenserver-core ones
are quite good.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #1680 from djs55/correct-list-address

Update the very old README