use inet_listen()/inet_connect() to support ipv6 migration
Use help functions in qemu-socket.c for tcp migration,
which already support ipv6 addresses.
Currently errp will be set to UNDEFINED_ERROR when migration fails,
qemu would output "migration failed: ...", and current user can
see a message("An undefined error has occurred") in monitor.
This patch changed tcp_start_outgoing_migration()/inet_connect()
/inet_connect_opts(), socket error would be passed back,
then current user can see a meaningful err message in monitor.
Qemu will exit if listening fails, so output socket error
to qemu stderr.
For IPv6 brackets must be mandatory if you require a port.
Referencing to RFC5952, the recommended format is:
[2312::8274]:5200