Evacuate api return migration

In the Masakari project (instance HA service), when one host failed, it
will evacuate all instances from the failure host. After it calls Nova
evacuate api, it will periodically check the instance 'vm_state' and
'hypervisor_hostname' to confirm whether the evacuation finished or
failed. And the confirmation is not simple and brief.

In nova evacuate api, it creates a migration objects whose status could
represent the evacuation progress. If Nove could return the migration
in the evacuate api, then it could help Masakari to refactor the
evacuation confirmation code.