I never use the net nic without some advanced options. See this for some advanced info.
and many more.
Where you need to state is at the client. If you delete the net nic you get the common NAT configuration for qemu.
Qemu has a virtual router software running. When you boot the client, you set it for dhcp and you should get an ip if you have nic support. (may need to set a new nic card or add in default nic driver)
Then I just add in my dns entry or in most cases it simply works. Your host works by it's choice of gateway or by availability.
Remove the net nic and then see ifconfig in client.