implement xen pv_irq_ops to paravirtualize irq handling
with xen event channel.

Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: NAkio Takebe <takebe_akio@jp.fujitsu.com>
Signed-off-by: NAlex Williamson <alex.williamson@hp.com>
Signed-off-by: NIsaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: NTony Luck <tony.luck@intel.com>