runqemu: allow bypassing of network setup

At present it is silently assumed all QEMU machines support networking.
As a consequence, one cannot run QEMUs without network emulation
using "runqemu".
This patch allows bypassing any network setup providing the qemuboot.conf
file contains:

    qb_net = none

[YOCTO#10661]

(From OE-Core rev: 6a9454027ced4efbb401a23df94f711b8253c8fa)

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -894,6 +894,8 @@ class BaseConfig(object):
         self.set('NETWORK_CMD', '%s %s' % (self.network_device.replace('@MAC@', mac), qemu_tap_opt))
 
     def setup_network(self):
+        if self.get('QB_NET') == 'none':
+            return
         cmd = "stty -g"
         self.saved_stty = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE).stdout.read().decode('utf-8')
         self.network_device = self.get('QB_NETWORK_DEVICE') or self.network_device