hw/i386/pc: Have pc_init_isa() pass a NULL pci_type argument

hw/i386/pc: Have pc_init_isa() pass a NULL pci_type argument

The "isapc" machine only provides an ISA bus, not a PCI one,
and doesn't instanciate any i440FX south bridge.
Its machine class defines PCMachineClass::pci_enabled = false,
and pc_init1() only uses the pci_type argument when pci_enabled
is true. Since for this machine the argument is not used,
passing NULL makes more sense.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20240301185936.95175-5-philmd@linaro.org>