migration: remove support for non-iovec based write handlers

All the remaining QEMUFile implementations provide an iovec
based write handler, so the put_buffer callback can be removed
to simplify the code.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1461751518-12128-28-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>