Recover from build state on compute manager start-up

If a compute manager is stopped / fails during a build operation
then the instance will be left stuck with vm_state=BUILDING

During restart the one thing we can be sure about such instances
is that providing the task state is not SCHEDULING (i.e the request
is not still on the queue) then there is no thread running for this
instance.

In this case we can treat instances in this state as if they have
failed to spawn and put the into an ERROR state.

Closes-Bug: 1197024
Related to blueprint recover-stuck-state

Change-Id: I7b116d8154036c43b0feb23f5669f52358408d2b
Merge "Recover from build state on compute manager start-up"