acpi_pcihp_eject_slot() bug if passed 'slots == 0'

[Peter Maydell]

Hi; Coverity spots that if hw/acpi/pcihp.c:acpi_pcihp_eject_slot()
is passed a zero 'slots' argument then ctz32(slots) will return 32,
and then the code that does '1U << slot' is C undefined behaviour
because it's an oversized shift. (This is CID 1421896.)

Since the pci_write() function in this file can call
acpi_pcihp_eject_slot() with an arbitrary value from the guest,
I think we need to handle 'slots == 0' safely. But what should
the behaviour be?

thanks
-- PMM