block/nfs: add support for nfs_umount

libnfs recently added support for unmounting. Add support
in Qemu too.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/nfs.c

@@ -398,6 +398,9 @@ static void nfs_client_close(NFSClient *client)
             nfs_close(client->context, client->fh);
             client->fh = NULL;
         }
+#ifdef LIBNFS_FEATURE_UMOUNT
+        nfs_umount(client->context);
+#endif
         nfs_destroy_context(client->context);
         client->context = NULL;
     }