libxc: xc_dom_parse_elf_kernel: Return error for invalid kernel images

libxc: xc_dom_parse_elf_kernel: Return error for invalid kernel images

Commit 96edb111dd ("libxc: panic when trying to create a PVH guest
without kernel support") already improved the handling of non PVH
capable kernels. But xc_dom_parse_elf_kernel() still returned success on
invalid elf images and the domain build only failed later. Now the build
process will fail immediately on detecting the error.

Signed-off-by: Simon Gaiser <simon@invisiblethingslab.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>