ip link add br0 type bridge
ip addr add 192.168.122.1/24 dev br0
ip link set br0 up

ip tuntap add mode tap tap0
ip link set tap0 master br0
ip link set tap0 up             // tap1 as well

qemu-system-x86_64 -enable-kvm -m 1G
-cdrom archlinux-2018.12.01-x86_64.iso
-drive file=vm.raw,format=raw,id=hd0
-device virtio-net,netdev=network0
-netdev tap,id=network0,ifname=tap0,script=no,downscript=no
-device virtio-net,netdev=network1
-netdev user,id=network1

iptables -A FORWARD -i br0 -o br0 -j ACCEPT
iptables -t nat -A POSTROUTING -s 192.168.122.0/24 -j MASQUERADE