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

Let the pc_build_feature_control_file() function take a generic MachineState
argument.

Suggested-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190818225414.22590-14-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>