migration: convert fd socket protocol to use QIOChannel

Convert the fd socket migration protocol driver to use
QIOChannel and QEMUFileChannel, instead of plain sockets
APIs. It can be unconditionally built because the
QIOChannel APIs it uses will take care to report suitable
error messages if needed.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1461751518-12128-16-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>