migration: convert unix socket protocol to use QIOChannel

Convert the unix socket migration protocol driver to use
QIOChannel and QEMUFileChannel, instead of plain sockets
APIs. It can be unconditionally built, since the socket
impl of QIOChannel will report a suitable error on platforms
where UNIX sockets are unavailable.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1461751518-12128-13-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>