KVM: Add KVM_MMIO kconfig item

s390 doesn't have mmio, this will simplify ifdefing it out.

Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index daece36..7f1178f 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -12,3 +12,6 @@
 
 config KVM_APIC_ARCHITECTURE
        bool
+
+config KVM_MMIO
+       bool