x86_iommu: check if machine has PCI bus

Starting qemu with
qemu-system-x86_64 -S -M isapc -device {amd|intel}-iommu
leads to a segfault. The code assume PCI bus is present and
tries to access the bus structure without checking.

Since Intel VT-d and AMDVI should only work with PCI, add a
check for PCI bus and return error if not present.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Mohammed Gamal <mgamal@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>

hw/i386/x86-iommu.c

@@ -88,7 +88,7 @@ static void x86_iommu_realize(DeviceState *dev, Error **errp)
         PC_MACHINE(object_dynamic_cast(OBJECT(ms), TYPE_PC_MACHINE));
     QLIST_INIT(&x86_iommu->iec_notifiers);
 
-    if (!pcms) {
+    if (!pcms || !pcms->bus) {
         error_setg(errp, "Machine-type '%s' not supported by IOMMU",
                    mc->name);
         return;