[S390] kvm-s390: fix kconfig dependencies

A user can create the Kconfig combination !VIRTUALIZATION, S390_GUEST
which results in the following warnings:

warning: (S390_GUEST) selects VIRTIO which has unmet direct dependencies (VIRTUALIZATION)
warning: (S390_GUEST && VIRTIO_PCI && VIRTIO_BALLOON) selects VIRTIO_RING which has unmet direct dependencies (VIRTUALIZATION && VIRTIO)
warning: (S390_GUEST) selects VIRTIO which has unmet direct dependencies (VIRTUALIZATION)
warning: (S390_GUEST && VIRTIO_PCI && VIRTIO_BALLOON) selects VIRTIO_RING which has unmet direct dependencies (VIRTUALIZATION && VIRTIO)

S390_GUEST has to select VIRTUALIZATION before selecting VIRTIO and
friends.
Reported-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -579,6 +579,7 @@ config S390_GUEST
 	def_bool y
 	prompt "s390 guest support for KVM (EXPERIMENTAL)"
 	depends on 64BIT && EXPERIMENTAL
+	select VIRTUALIZATION
 	select VIRTIO
 	select VIRTIO_RING
 	select VIRTIO_CONSOLE