S390: fix error handling on kernel and initrd failures If the user specifies a non-existing or non-accessable kernel or initrd qemu does not fail, instead it ipls into the system, which then falls into a program check loop due to the zeroed memory with no kernel. Lets add some sanity checks. Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com> Signed-off-by: Alexander Graf <agraf@suse.de>