hw: make virtio devices configurable via default-configs/

This is only half of the work, because the proxy devices (virtio-*-pci,
virtio-*-ccw, etc.) are still included unconditionally.  It is still a
move in the right direction.

Based-on: <20180522194943.24871-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/hw/scsi/Makefile.objs b/hw/scsi/Makefile.objs
index b188f72..718b4c2 100644
--- a/hw/scsi/Makefile.objs
+++ b/hw/scsi/Makefile.objs
@@ -8,7 +8,7 @@
 common-obj-$(CONFIG_ESP_PCI) += esp-pci.o
 obj-$(CONFIG_PSERIES) += spapr_vscsi.o
 
-ifeq ($(CONFIG_VIRTIO),y)
+ifeq ($(CONFIG_VIRTIO_SCSI),y)
 obj-y += virtio-scsi.o virtio-scsi-dataplane.o
 obj-$(CONFIG_VHOST_SCSI) += vhost-scsi-common.o vhost-scsi.o
 obj-$(CONFIG_VHOST_USER_SCSI) += vhost-scsi-common.o vhost-user-scsi.o