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>
