pc: apic_common: Reset APIC ID to initial ID when switching into x2APIC mode

SDM: x2APIC State Transitions:
         State Changes From xAPIC Mode to x2APIC Mode
"
Any APIC ID value written to the memory-mapped
local APIC ID register is not preserved
"

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

hw/intc/apic_common.c

@@ -40,6 +40,11 @@ void cpu_set_apic_base(DeviceState *dev, uint64_t val)
     if (dev) {
         APICCommonState *s = APIC_COMMON(dev);
         APICCommonClass *info = APIC_COMMON_GET_CLASS(s);
+        /* switching to x2APIC, reset possibly modified xAPIC ID */
+        if (!(s->apicbase & MSR_IA32_APICBASE_EXTD) &&
+            (val & MSR_IA32_APICBASE_EXTD)) {
+            s->id = s->initial_apic_id;
+        }
         info->set_base(s, val);
     }
 }