docs etc.: https: Fix references to other Xen pages

Change the url scheme to https.  This is all in-tree references to
xenbits and the main website except for those in Config.mk.

We leave Config.mk alone for now because those urls are used by CI
systems and we need to check that nothing breaks when we change the
download method.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>

README

@@ -7,7 +7,7 @@
 
 #################################
 
-http://www.xen.org/
+https://www.xen.org/
 
 What is Xen?
 ============
@@ -17,10 +17,10 @@ Systems Research Group of the University of Cambridge Computer
 Laboratory, as part of the UK-EPSRC funded XenoServers project.  Xen
 is freely-distributable Open Source software, released under the GNU
 GPL. Since its initial public release, Xen has grown a large
-development community, spearheaded by xen.org (http://www.xen.org).
+development community, spearheaded by xen.org (https://www.xen.org).
 
 This file contains some quick-start instructions to install Xen on
-your system. For more information see http:/www.xen.org/ and
+your system. For more information see https:/www.xen.org/ and
 https://wiki.xen.org/
 
 Quick-Start Guide
@@ -99,7 +99,7 @@ performed with root privileges.]
 1. Download and untar the source tarball file. This will be a
    file named xen-unstable-src.tgz, or xen-$version-src.tgz.
    You can also pull the current version from the git or mercurial
-   repositories at http://xenbits.xen.org/
+   repositories at https://xenbits.xen.org/
 
     # tar xzf xen-unstable-src.tgz