hw/i386/pc: Extract the x86 generic fw_cfg code

Extract all the functions that are not PC-machine specific into
the (arch-specific) fw_cfg.c file. This will allow other X86-machine
to reuse these functions.

Suggested-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190818225414.22590-16-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>