Some important parts of f6df72e7 got
dropped along the way, reintroduce them.

Only affects paravirt guests.
Signed-off-by: NIan Campbell <ijc@hellion.org.uk>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>