Moves scheduler.manager._set_vm_state_and_notify to scheduler.utils

This method is required by the scheduler.manager.prep_resize method
which is being moved to conductor.manager.

We move it to scheduler.utils until we completely move migrations
logic out of scheduler.manager.

Partially implements bp cold-migrations-to-conductor

Change-Id: I8203577fc26da5cc1039815e31fb8fe356d1f7d6
Merge "Moves scheduler.manager._set_vm_state_and_notify to scheduler.utils"