[IA64] Cleanup to irq.c

[IA64] Cleanup to irq.c

This patch is a big cleanup to irq.c, with only real
necessary interfaces kept now. The irq subsystem of
xen is very simple since most external devices are
controlled by dom0. Also especially there's no need
to keep CONFIG_XEN everywhere, since this file is
already out of sync of linux version for a long time.
Actually xen defines irq related structures differently
and it's worthy of creating a xen specific version here.

Another change of this patch is to remove ugly check
upon timer/ipi within irq handler. Instead now we'll
check IRQ_GUEST flag of irq_desc to decide whether
to deliver irq to guest. Now all vectors un-registered
are set with IRQ_GUEST enabled, which is identical to
current assumption. Later this flag should be set only
per guest's request. Then Xen_do_irq is not reuqired
as the result of this change.

Signed-off-by: Kevin Tian <kevin.tian@intel.com>