devshell.bbclass: Allow devshell & pydevshell to use the network

Otherwise it will fail if using OE_TERMINAL = "xterm" with the not so
helpful error:

  xterm: Xt error: Can't open display: localhost:0.0

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit ba53fc3b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>