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: NGonglei <arei.gonglei@huawei.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>