migration: Disable postcopy + multifd migration

migration: Disable postcopy + multifd migration

Since the introduction of multifd, it's possible to perform a multifd
migration and finish it using postcopy.

A bug introduced by yank (fixed on cfc3bcf373) was previously preventing
a successful use of this migration scenario, and now thing should be
working on most scenarios.

But since there is not enough testing/support nor any reported users for
this scenario, we should disable this combination before it may cause any
problems for users.

Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Leonardo Bras <leobras@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>