virtio-scsi: fix disabled mode

Add two missing checks for s->dataplane_fenced.  In one case, QEMU
would skip injecting an IRQ due to a write to an uninitialized
EventNotifier's file descriptor.

In the second case, the dataplane_disabled field was used by mistake;
in fact after fixing this occurrence it is completely unused.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
