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>

nbd.c

@@ -721,7 +721,6 @@ void nbd_export_close(NBDExport *exp)
         g_free(first);
     }
 
-    bdrv_close(exp->bs);
     g_free(exp);
 }
 

qemu-nbd.c

@@ -586,6 +586,7 @@ int main(int argc, char **argv)
     } while (!sigterm_reported && (persistent || !nbd_started || nb_fds > 0));
 
     nbd_export_close(exp);
+    bdrv_close(bs);
     if (sockpath) {
         unlink(sockpath);
     }