vhost: check region type before casting

Check region type first before casting the memory region
to IOMMUMemoryRegion. Otherwise QEMU will abort with below
error message when casting non-IOMMU memory region:

vhost_iommu_region_add: Object 0x561f28bce4f0 is not an
instance of type qemu:iommu-memory-region

Fixes: cb1efcf4 ("iommu: Add IOMMU index argument to notifier APIs")
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: NTiwei Bie <tiwei.bie@intel.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost.c

@@ -663,12 +663,14 @@ static void vhost_iommu_region_add(MemoryListener *listener,
     struct vhost_iommu *iommu;
     Int128 end;
     int iommu_idx;
-    IOMMUMemoryRegion *iommu_mr = IOMMU_MEMORY_REGION(section->mr);
+    IOMMUMemoryRegion *iommu_mr;
 
     if (!memory_region_is_iommu(section->mr)) {
         return;
     }
 
+    iommu_mr = IOMMU_MEMORY_REGION(section->mr);
+
     iommu = g_malloc0(sizeof(*iommu));
     end = int128_add(int128_make64(section->offset_within_region),
                      section->size);