xen: make sure retriggered events are set pending

retrigger_dynirq() was incomplete, and didn't properly set the event
to be pending again.  It doesn't seem to actually get used.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>