Make restore_instance pass the Instance object to compute manager

The compute/api code already receives and uses an object, but
converts it to a dict before sending it down. This plumbs the
instance object all the way through to the driver.

This also converts the old compute_api non-unity test to a
proper unit test in test_compute_api, per usual when converting
API calls to objects.

Related to blueprint compute-manager-objects
Related to blueprint virt-objects

Change-Id: Ic2d7d14c50d85556ed1170f7d026c9d84423c876
Merge "Make restore_instance pass the Instance object to compute manager"