migration: unify the framework of socket-type channel

Currently, the only difference of tcp channel and unix channel in
migration/socket.c is the way to build SocketAddress, but socket_parse()
can handle these two types, so use it to instead of tcp_build_address()
and unix_build_address().

The socket-type channel can be further unified based on the up, this
would be helpful for us to add other socket-type channels.

Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Message-Id: <20200806074030.174-2-longpeng2@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>