Make /etc/rc.d/rc.ip_forward executable. The command below is unneeded since reboot.

/etc/rc.d/rc.ip_forward start

Create /usr/local/bin/qemu-ifup

/usr/sbin/brctl addbr br0
/usr/sbin/ip addr add 172.20.0.1/16 dev br0
/usr/sbin/ip link set br0 up
/usr/sbin/dnsmasq --interface=br0 --bind-interfaces --dhcp-range=172.20.0.2,172.20.255.254
/usr/sbin/tunctl -t tap0 -u 500
/usr/sbin/ip link set tap0 up promisc on
/usr/sbin/ip link set tap0 master br0
/usr/sbin/iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE
/usr/sbin/iptables -A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
/usr/sbin/iptables -A FORWARD -i tap0 -o wlan0 -j ACCEPT

Create /usr/local/bin/qemu-ifdown

/usr/sbin/iptables -F
/sbin/ifconfig tap0 down
/usr/sbin/tunctl -d tap0
pkill dnsmasq
/sbin/ifconfig br0 down
/usr/sbin/brctl delbr br0

The script for running a virtual machine.

Replace diskimage.qcow2 and cdimage.iso with actual filenames of hard disk and cdrom images.

If the guest went black after startup, remove -display gtk,gl=on

sudo /usr/local/bin/qemu-ifup
qemu-system-x86_64 -enable-kvm -machine q35 -device intel-iommu -cpu host -m 4G -vga virtio -display gtk,gl=on -audio driver=pa,model=virtio -net nic -net tap,ifname=tap0,script=no,downscript=no -drive file=diskimage.qcow2,format=qcow2 -cdrom cdimage.iso
sudo /usr/local/bin/qemu-ifdown