x86: defer processing events on the NMI exit path

x86: defer processing events on the NMI exit path

Otherwise, we may end up in the scheduler, keeping NMIs masked for a
possibly unbounded period of time (until whenever the next IRET gets
executed). Enforce timely event processing by sending a self IPI.

Of course it's open for discussion whether to always use the straight
exit path from handle_ist_exception.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>