x86_64: mbi-cmdline is a 4 byte addr

Treating mbi-cmdline as an 8 byte addr includes mbi-mods-count,
which only works as long as there are no mods.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>