network address in vm by kvm

[Max Power]

Hi guys,
I need to configure my Virtual Host [OpenBSD 6.0 amd64]
in the way [bottom: Linux example]:

auto eth0
iface eth0 inet static
        address 5.6.77.8
        netmask 255.255.255.255
        broadcast 5.6.77.8
        post-up route add 123.4.5.254 dev eth0
        post-up route add default gw 123.4.5.254
        post-down route del default gw 123.4.5.254
        post-down route del 123.4.5.254 dev eth0

Can someone tell me how to do?

Thanks for reply.