intel_iommu: vtd_slpt_level_shift check level

This helps in debugging incorrect level passed in.
Reviewed-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Reviewed-by: NDavid Gibson <david@gibson.dropbear.id.au>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/i386/intel_iommu.c

@@ -168,6 +168,7 @@ static gboolean vtd_hash_remove_by_domain(gpointer key, gpointer value,
 /* The shift of an addr for a certain level of paging structure */
 static inline uint32_t vtd_slpt_level_shift(uint32_t level)
 {
+    assert(level != 0);
     return VTD_PAGE_SHIFT_4K + (level - 1) * VTD_SL_LEVEL_BITS;
 }