KVM: s390: Fix guest kconfig

Cornelia Huck noticed that a modular virtio without kvm guest support
leads to a build error in the s390 virtio transport:

CONFIG_VIRTIO=m leads to
ERROR: "vmem_add_mapping" [drivers/s390/kvm/kvm_virtio.ko] undefined!
ERROR: "max_pfn" [drivers/s390/kvm/kvm_virtio.ko] undefined!
ERROR: "vmem_remove_mapping" [drivers/s390/kvm/kvm_virtio.ko] undefined!

The virtio transport only works with kvm guest support and only as a
builtin. Lets change the build process of drivers/s390/kvm/kvm_virtio.c
to depend on kvm guest support, which is also a bool.

CONFIG_S390_GUEST already selects CONFIG_VIRTIO, that should prevent
CONFIG_S390_GUEST=y CONFIG_VIRTIO=n situations.

CC: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>