QEMU fw_cfg: Write fw_cfg back on S3 resume

Any pointers to BIOS-allocated memory that were written back to QEMU
fw_cfg files are replayed when resuming from S3 sleep.

Signed-off-by: Ben Warren <ben@skyportsystems.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
(cherry picked from commit 8f598a46)