intel_iommu: simplify irq region translation

Now we have a standalone memory region for MSI, all the irq region
requests should be redirected there. Cleaning up the block with an
assertion instead.

Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>