io: set LISTEN flag explicitly for listen sockets

io: set LISTEN flag explicitly for listen sockets

The SO_ACCEPTCONN ioctl is not portable across OS, with
some BSD versions and OS-X not supporting it. There is
no viable alternative to this, so instead just set the
feature explicitly when creating a listener socket.

The current users of qio_channel_socket_new_fd() won't
ever be given a listening socket, so there's no problem
with no auto-detecting it in this scenario

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>