scripts/runqemu: Add possibility to disable network

scripts/runqemu: Add possibility to disable network

Default network configuration requires tun/tap module and while being
usable it conflicts with tap devices created by VPN clients sometimes
and requires root permissions to use . While it's possible to work
this around it's not always feasible if network is not required
Add nonetwork option which can be specified if the network connectivity is
not needed and SDL/serial is enough to communicate with the image.

Signed-off-by: Pavel Zhukov <pazhukov@suse.de>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>