CA-139894: Fix waiting for VM.remove

Due to a change in semantics of UPDATES.get, task_wait would
occasionally not wait.

This also adds back in error handling into the VM.remove path.

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Add a comment showing why an assert is there.

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Merge pull request #83 from jonludlam/CA-139894

CA-139894