x86-iommu: provide x86_iommu_get_default

Instead of searching the device tree every time, one static variable is
declared for the default system x86 IOMMU device.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
index fbba461..12ecf95 100644
--- a/hw/i386/acpi-build.c
+++ b/hw/i386/acpi-build.c
@@ -52,7 +52,7 @@
 #include "hw/i386/ich9.h"
 #include "hw/pci/pci_bus.h"
 #include "hw/pci-host/q35.h"
-#include "hw/i386/intel_iommu.h"
+#include "hw/i386/x86-iommu.h"
 #include "hw/timer/hpet.h"
 
 #include "hw/acpi/aml-build.h"
@@ -2539,12 +2539,7 @@
 
 static bool acpi_has_iommu(void)
 {
-    bool ambiguous;
-    Object *intel_iommu;
-
-    intel_iommu = object_resolve_path_type("", TYPE_INTEL_IOMMU_DEVICE,
-                                           &ambiguous);
-    return intel_iommu && !ambiguous;
+    return !!x86_iommu_get_default();
 }
 
 static