migration: Export migrate_set_state()

migration: Export migrate_set_state()

Change the first parameter of migrate_set_state(), and export it.
We will use it in a later patch to update incoming state.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
dgilbert: Updated comment as per Juan's review
Message-Id: <1450266458-3178-2-git-send-email-dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>