For basic tests it's not necessary to have the networking setup and this
allows testing as a normal user as well, without root privileges.
So this patch adds `--no-network` long option or `-n` short option,
which allows starting QEMU without network.
Cc: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
