s390/kvm: Improve Kconfig help text.

CONFIG_S390_GUEST determines whether virtio guest drivers are built, and
the virtio console is not enforced as default.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>