virtio-scsi: fix dataplane

Commit 361dcc7 (virtio-scsi: dataplane: fail setup gracefully, 2014-10-15)
actually broke successful dataplane setup in a not-so-graceful manner:

    qemu-system-x86_64: .../util/rfifolock.c:71: rfifolock_unlock: Assertion `r->nesting > 0' failed.

due to a missing return statement.

Fixes: 361dcc790d
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/virtio-scsi-dataplane.c

@@ -241,9 +241,10 @@ void virtio_scsi_dataplane_start(VirtIOSCSI *s)
         }
     }
 
-    aio_context_release(s->ctx);
     s->dataplane_starting = false;
     s->dataplane_started = true;
+    aio_context_release(s->ctx);
+    return;
 
 fail_vrings:
     virtio_scsi_clear_aio(s);