Fix for poweroff issue (bug #994899)

I've build packages with these patches included and my Xen server shuts down properly with that.

We had already identified 3 commits which we later found to all belong to 1 patch-set, so I took the whole patch-set.
Also identified another commit which was a (further) fix to that patch-set, so I included that as well.

Also added a changelog entry to give it a unique version name so I could verify that I was actually running that version and to document where those patches came from. This is a rebased version of !13 (closed) onto master.
Rebuilding for bullseye-security will be left for the maintainers.