net: update netdev stream man page with unix socket

net: update netdev stream man page with unix socket

Add the description of "-netdev stream" with a unix domain socket.
The code has been added but the man page has not been updated.

Include an example how to use "-netdev stream" and "passt" in place
of "-netdev user".
("passt" is a non privileged translation proxy between layer-2, like
"-netdev stream", and layer-4 on host, like TCP, UDP, ICMP/ICMPv6 echo)

Fixes: 13c6be96618c ("net: stream: add unix socket")
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>