Cells: Fix instance deletes

A regression was introduced with object conversion that caused all
instances to be locally deleted in the API cell when cells is enabled.
This would happen before the child cell was told to delete the instance.

Additionally, if an instance record was created in a child cell but
failed to schedule to a host in such a way that the API cell has
no knowledge of the 'host', the instance would be deleted in the API
cell only.

Also, it appears that we would cast deletes to child cells twice due to
logic duplicated in cells_api.py that exists in api.py.

Closes-Bug: #1240247

Change-Id: I94cfddd9cef2d63b2cb31c0f2dae8e211a4418d9
Merge "Cells: Fix instance deletes"