xio3130_downstream: Report error if pcie_chassis_add_slot() failed

On commit f8cd1b02 ("pci: Convert to realize"), no error_set*()
call was added for the pcie_chassis_add_slot() error case.
pcie_chassis_add_slot() errors get ignored, making QEMU crash
later.  e.g.:

  $ qemu-system-x86_64 -device ioh3420 -device xio3130-downstream
  qemu-system-x86_64: memory.c:2166: memory_region_del_subregion: Assertion `subregion->container == mr' failed.
  Aborted (core dumped)

Fix it by reporting the error using error_setg().

Fixes: f8cd1b02Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Reviewed-by: NMarcel Apfelbaum <marcel@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/pci-bridge/xio3130_downstream.c

@@ -94,6 +94,7 @@ static void xio3130_downstream_realize(PCIDevice *d, Error **errp)
     pcie_chassis_create(s->chassis);
     rc = pcie_chassis_add_slot(s);
     if (rc < 0) {
+        error_setg(errp, "Can't add chassis slot, error %d", rc);
         goto err_pcie_cap;
     }