Comment 1 for bug 1046313

It's possible that umount returns before actually completing the umount
(communication with the qemu-nbd processes etc.). Though that would
be a bug (as we don't specify the -z option). Now there is a similar bug in
FUSE that we had to work around in the libguestfs implementation, so it's possible.

What kernel version/distro are you using?

Also I'm a bit confused that when you ran the mount command,
the mount point showed up. If it was a race, then mount point
would probably be unmounted before you ran that command.
There are definitely no mentions of the following in the log?
  "Failed to unmount container filesystem"

If not and this only happens intermittently, then
logically either umount silently fails somethimes with nbd,
or sometimes the _device_for_path() function in
/usr/local/nova/nova/virt/disk/api.py doesn't find the corresponding device.
Both surprising TBH.