hw/pci/msix: Warn on PBA writes

hw/pci/msix: Warn on PBA writes

Of the MSI-X PBA pending bits, the PCI Local Bus Specification says:

  Software should never write, and should only read
  Pending Bits. If software writes to Pending Bits, the
  result is undefined.

Log a GUEST_ERROR message if the PBA is written to by software.

Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Cc: Dmitry Fleytman <dmitry.fleytman@gmail.com>
Cc: Sriram Yagnaraman <sriram.yagnaraman@ericsson.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20250117172842.406338-2-npiggin@gmail.com>
Reviewed-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>