CONFIG_S390_GUEST is not available for thedefaultarchitecture x86.
Result is shown for architecture 64BIT or s390

s390 support for virtio devices

configname: CONFIG_S390_GUEST

Linux Kernel Configuration
└─>Virtualization
└─>s390 support for virtio devices
In linux kernel since version 2.6.26 (release Date: 2008-07-13)  
Enabling this option adds support for virtio based paravirtual device
drivers on s390.

Select this option if you want to run the kernel as a guest under
the KVM hypervisor.