multiboot: Prevent loading of x86_64 images

A via -kernel supplied x86_64 ELF image is being started in 32bit mode.
Detect and exit if a 64bit image has been supplied.

Signed-off-by: Adam Lackorzynski <adam@os.inf.tu-dresden.de>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
