virtio-balloon: dependency fix

Devices should depend on virtio, not select it.  It's supposed to be
selected by the particular driver, e.g. VIRTIO_PCI.
Make balloon depend on VIRTIO and EXPERIMENTAL
(to match description).
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/Kconfig

@@ -27,8 +27,7 @@ config VIRTIO_PCI
 
 config VIRTIO_BALLOON
 	tristate "Virtio balloon driver (EXPERIMENTAL)"
-	select VIRTIO
-	select VIRTIO_RING
+	depends on EXPERIMENTAL && VIRTIO
 	---help---
 	 This driver supports increasing and decreasing the amount
 	 of memory within a KVM guest.