nbd: do not close BlockDriverState in nbd_export_close

This is not desirable when embedding the NBD server inside QEMU.
Move the bdrv_close to qemu-nbd.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 1c1cf6a..23392e0 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -586,6 +586,7 @@
     } while (!sigterm_reported && (persistent || !nbd_started || nb_fds > 0));
 
     nbd_export_close(exp);
+    bdrv_close(bs);
     if (sockpath) {
         unlink(sockpath);
     }