acpi: validate hotplug selector on access

acpi: validate hotplug selector on access

When bus is looked up on a pci write, we didn't
validate that the lookup succeeded.
Fuzzers thus can trigger QEMU crash by dereferencing the NULL
bus pointer.

Fixes: b32bd763a1 ("pci: introduce acpi-index property for PCI device")
Fixes: CVE-2021-4158
Cc: "Igor Mammedov" <imammedo@redhat.com>
Fixes: https://gitlab.com/qemu-project/qemu/-/issues/770
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Ani Sinha <ani@anisinha.ca>