xen: definitions which ia64/xen needs

Add xen VIRQ numbers defined for arch specific use.
ia64/xen domU uses VIRQ_ARCH_0 for virtual itc timer.
Although all those constants aren't used yet by ia64
at this moment, add all arch specific VIRQ numbers.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
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>