apic: fix incorrect handling of ExtINT interrupts wrt processor priority

This fixes another failure with ExtINT, demonstrated by QNX.  The failure
mode is as follows:
- IPI sent to cpu 0 (bit set in APIC irr)
- IPI accepted by cpu 0 (bit cleared in irr, set in isr)
- IPI sent to cpu 0 (bit set in both irr and isr)
- PIC interrupt sent to cpu 0

The PIC interrupt causes CPU_INTERRUPT_HARD to be set, but
apic_irq_pending observes that the highest pending APIC interrupt priority
(the IPI) is the same as the processor priority (since the IPI is still
being handled), so apic_get_interrupt returns a spurious interrupt rather
than the pending PIC interrupt. The result is an endless sequence of
spurious interrupts, since nothing will clear CPU_INTERRUPT_HARD.

Instead, ExtINT interrupts should have ignored the processor priority.
Calling apic_check_pic early in apic_get_interrupt ensures that
apic_deliver_pic_intr is called instead of delivering the spurious
interrupt.  apic_deliver_pic_intr then clears CPU_INTERRUPT_HARD if needed.

Reported-by: Richard Bilson <rbilson@qnx.com>
Tested-by: Richard Bilson <rbilson@qnx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/intc/apic.c

@@ -571,7 +571,10 @@ int apic_get_interrupt(DeviceState *dev)
     apic_sync_vapic(s, SYNC_FROM_VAPIC);
     intno = apic_irq_pending(s);
 
-    if (intno == 0) {
+    /* if there is an interrupt from the 8259, let the caller handle
+     * that first since ExtINT interrupts ignore the priority.
+     */
+    if (intno == 0 || apic_check_pic(s)) {
         apic_sync_vapic(s, SYNC_TO_VAPIC);
         return -1;
     } else if (intno < 0) {
@@ -582,9 +585,6 @@ int apic_get_interrupt(DeviceState *dev)
     apic_set_bit(s->isr, intno);
     apic_sync_vapic(s, SYNC_TO_VAPIC);
 
-    /* re-inject if there is still a pending PIC interrupt */
-    apic_check_pic(s);
-
     apic_update_irq(s);
 
     return intno;