This reverts commit 4da97120d5.
blk_aio_flush() now handles the blk->root == NULL case, so we no longer
need this workaround.
Cc: John Snow <jsnow@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>