block: Call external_snapshot_clean after blockdev-snapshot

Otherwise the AioContext will never be released.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Message-id: 1447419624-21918-1-git-send-email-berto@igalia.com
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

blockdev.c

@@ -2098,6 +2098,7 @@ static const BlkActionOps actions[] = {
         .prepare  = external_snapshot_prepare,
         .commit   = external_snapshot_commit,
         .abort = external_snapshot_abort,
+        .clean = external_snapshot_clean,
     },
     [TRANSACTION_ACTION_KIND_BLOCKDEV_SNAPSHOT_SYNC] = {
         .instance_size = sizeof(ExternalSnapshotState),