hw/i386/pc: Extract e820 memory layout code

Suggested-by: Samuel Ortiz <sameo@linux.intel.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190818225414.22590-3-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>