fw_cfg: fix boot order bug when dynamically modified via QOM
When we dynamically modify boot order, the length of
boot order will be changed, but we don't update
s->files->f[i].size with new length. This casuse
seabios read a wrong vale of qemu cfg file about
bootorder.
Cc: Gerd Hoffmann <kraxel@redhat.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Gonglei <arei.gonglei@huawei.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>