xen/events: refactor retrigger_dynirq() and resend_irq_on_evtchn()

These two function did the same thing with different parameters, put
the common bits in retrigger_evtchn().

This changes the return value of resend_irq_on_evtchn() but the only
caller (in arch/ia64/xen/irq_xen.c) ignored the return value so this
is fine.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>