You can configure an IP from /27 subnet on the eth0/8 OR on eth0/9, and use this as default gateway on your VMs. Once traffic from /27 IP reaches to the device, it will do the route lookup and forward the traffic.
If you want to use both the ports Eth0/8 and Eth0/9, then how will you connect it to the VMs or to the KVM server? If it's switching environment then you can try configuring the bgroup( kind of layer3 VLAN). Check https://help.juniper.net/help/english/6.3.0/if_edit_cnt.htm for more details.
Thanks,
Vikas