migration: delete QEMUFile sockets implementation

Now that the tcp, unix and fd migration backends have converted
to use the QIOChannel based QEMUFile, there is no user remaining
for the sockets based QEMUFile impl and it can be deleted.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1461751518-12128-22-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>