ioapic: fix contents of arbitration register

The arbitration register should read to the same value as the
IOAPIC id register.  Fixes kvm-unit-tests ioapic.flat.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/intc/ioapic.c

@@ -156,15 +156,13 @@ ioapic_mem_read(void *opaque, hwaddr addr, unsigned int size)
         }
         switch (s->ioregsel) {
         case IOAPIC_REG_ID:
+        case IOAPIC_REG_ARB:
             val = s->id << IOAPIC_ID_SHIFT;
             break;
         case IOAPIC_REG_VER:
             val = IOAPIC_VERSION |
                 ((IOAPIC_NUM_PINS - 1) << IOAPIC_VER_ENTRIES_SHIFT);
             break;
-        case IOAPIC_REG_ARB:
-            val = 0;
-            break;
         default:
             index = (s->ioregsel - IOAPIC_REG_REDTBL_BASE) >> 1;
             if (index >= 0 && index < IOAPIC_NUM_PINS) {