migration: rename unix.c to socket.c

The unix.c file will be nearly the same as the tcp.c file,
only differing in the initial SocketAddress creation code.
Rename unix.c to socket.c and refactor it a little to
prepare for merging the TCP code.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1461751518-12128-14-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>