Add Instance.AvailabilityZone.
Add instanceData.AvailZone.
Add state-related utility functions.
Add an upgrade step for availability zones in instance data.
Drop Instance.AvailabilityZone.
Fix some typos.
Drop unnecessary code.
Add Machine.AvailabilityZone.
Update provider (and related) tests.
Clean up an error message.
Fix a couple bugs in AddAvailabilityZoneToInstanceData.
Add a test for AddAvailabilityZoneToInstanceData.
Add tests for the state utils.
Add upgrade-related tests.
Use errors.Trace.
txn.Op is implicit.
Use errors.Trace.
Use errors.Trace.
gc.IsNil -> jc.ErrorIsNil.
Drop blank line.