hw/pci-bridge: pci_expander_bridge fix type in pxb_cxl_dev_reset()

hw/pci-bridge: pci_expander_bridge fix type in pxb_cxl_dev_reset()

Reproduce issue with

configure --enable-qom-cast-debug ...

qemu-system-x86_64 -display none -machine q35,cxl=on -device pxb-cxl,bus=pcie.0

  hw/pci-bridge/pci_expander_bridge.c:54:PXB_DEV: Object 0x5570e0b1ada0 is not an instance of type pxb
  Aborted

The type conversion results in the right state structure, but PXB_DEV is
not a parent of PXB_CXL_DEV hence the error. Rather than directly
cleaning up the inheritance, this is the minimal fix which will be
followed by the cleanup.

Fixes: 154070eaf6 ("hw/pxb-cxl: Support passthrough HDM Decoders unless overridden")
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20230420142750.6950-2-Jonathan.Cameron@huawei.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>