hw/cxl: Fix msix_notify: Assertion `vector < dev->msix_entries_nr`

hw/cxl: Fix msix_notify: Assertion `vector < dev->msix_entries_nr`

This assertion always happens when we sanitize the CXL memory device.
$ echo 1 > /sys/bus/cxl/devices/mem0/security/sanitize

It is incorrect to register an MSIX number beyond the device's capability.

Increase the device's MSIX number to cover the mailbox msix number(9).

Fixes: 43efb0bfad2b ("hw/cxl/mbox: Wire up interrupts for background completion")
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Message-Id: <20250115075834.167504-1-lizhijian@fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>