ioapic: Do not set irr for masked edge IRQs

So far we set IRR for edge IRQs even if the pin is masked. If the guest
later on unmasks and switches the pin to level-triggered mode, irr will
remain set, causing an IRQ storm. The point is that setting IRR is not
correct in this case according to the spec, and avoiding this resolves
the issue.
Reported-and-tested-by: NIsaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

hw/ioapic.c

@@ -160,8 +160,9 @@ static void ioapic_set_irq(void *opaque, int vector, int level)
                 s->irr &= ~mask;
             }
         } else {
-            /* edge triggered */
-            if (level) {
+            /* According to the 82093AA manual, we must ignore edge requests
+             * if the input pin is masked. */
+            if (level && !(entry & IOAPIC_LVT_MASKED)) {
                 s->irr |= mask;
                 ioapic_service(s);
             }