hw/i386/pc: Pass the boot_cpus value by argument

The boot_cpus is used once. Pass it by argument, this will
allow us to remove the PCMachineState argument later.

Suggested-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190818225414.22590-6-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>