Big fixes for the new IO-APIC acknowledging method. The problems

Big fixes for the new IO-APIC acknowledging method. The problems
were:
 1. Some critical Xen interrupts could get blocked behind
    unacknowledged guest interrupts. This is avoided by making
    all Xen-bound interrrupts strictly higher priority.
 2. Interrupts must not only be EOIed on the CPU that received
    them, but also in reverse order when interrupts are nested.
    A whole load of logic has been added to ensure this.

There are two boot parameters relating to all this:
 'ioapic_ack=old' -- use the old IO-APIC ACK method
 'ioapic_ack=new' -- use the new IO-APIC ACK method (default)
 'force_intack'   -- periodically force acknowledgement of
                     interrupts (default is no; useful for debugging)

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