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: f8cd1b0201c41d88bb97dcafb80348a0e88d8805
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
