qcow2: Fix snapshot restoration in snapshot_create

If the new snapshot table could not be written in qcow2_snapshot_create,
the old snapshot table has to be restored in memory and the new one
released. This should include restoration of the old snapshot count as
well, which is added by this patch.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-snapshot.c

@@ -433,6 +433,7 @@ int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
     if (ret < 0) {
         g_free(s->snapshots);
         s->snapshots = old_snapshot_list;
+        s->nb_snapshots--;
         goto fail;
     }