Fix Xen's interrupt acknowledgement routines on certain

Fix Xen's interrupt acknowledgement routines on certain
(apparently broken) IO-APIC hardware:
 1. Do not mask/unmask the IO-APIC pin during normal ISR
    processing. This seems to have really bizarre side effects
    on some chipsets.
 2. Since we instead tickle the local APIC in the ->end
    irq hook function, it *must* run on the CPU that
    received the interrupt. Therefore we track which CPUs
    need to do final acknowledgement and IPI them if
    necessary to do so.

Signed-off-by: Keir Fraser <keir@xensource.com>