Target host in evacuate can't be the original one

'Evacuate' function aims to help administrator/operator to evacuate
servers if this compute node fails.

If no protection, the API returns a successful response,
but the vm_state will be changed to ERROR after nova-compute started,
due to the 'InstanceExists' exception raised from 'rebuild_instance()'.

So we need to add verification here, the target host should not be the
original host.

Change-Id: Ic468cd57688b370a18cacfc6e0844a8201eb9ab3
Closes-Bug: #1271821
Merge "Target host in evacuate can't be the original one"