s390x: fix KVM target

During Jan's rework of the generic KVM layer, he added some more error checks
and actually aborted if something went wrong. Unfortunately, one of the s390
internal error codes slipped through, aborting the VM without needing to.

This patch fixes booting of S390x virtual machines in KVM.

Signed-off-by: Alexander Graf <agraf@suse.de>
CC: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
