hw/i386/pc: Let pc_build_feature_control() take a FWCfgState argument

Pass the FWCfgState object 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: <20190818225414.22590-13-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>