spapr: support only ELF kernel images

Currently everybody uses ELF kernel images with "-kernel" option on
pseries machine but QEMU still tries to boot from an image even it
fails to recognize it is ELF. This produces undefined behaviour if
the user tries a kernel image compiled for another architecture.

This removes support of raw kernel images.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
