build: Use HTTPS for all xenbits.xen.org Git repos

Obtaining code over an insecure transport is a terrible idea for
blatently obvious reasons.  Even for non-executable data, insecure
transports are considered deprecated.

This patch enforces the use of secure transports for all xenbits git
repositories.  It was generated with the following shell script:

    git ls-files -z |
    xargs -0 -- sed -Ei -- 's@(git://xenbits\.xen\.org|http://xenbits\.xen\.org/git-http)/@https://xenbits.xen.org/git-http/@g'

All altered links have been tested and are known to work.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

docs/process/xen-release-management.pandoc

@@ -271,7 +271,7 @@ Hi all,
 
 Xen X.Y rcZ is tagged. You can check that out from xen.git:
 
-git://xenbits.xen.org/xen.git X.Y.0-rcZ
+https://xenbits.xen.org/git-http/xen.git X.Y.0-rcZ
 
 For your convenience there is also a tarball at:
 https://downloads.xenproject.org/release/xen/X.Y.0-rcZ/xen-X.Y.0-rcZ.tar.gz