The Yocto Project's qemu integration is quite fascinating. It allows you to build an image targetting a qemu machine, then run it with their runqemu command. Various Yocto magic comes together to run the image under qemu, including getting the networking right so you can access the target from your host.
If all you need to do with a qemu image, networking-wise, is to reach out and do something over tcp (e.g. surf the web, install updates, etc) then qemu's "-net user" mode is more than adequate. One of the great features of the "-net user" mode is that it doesn't require root permissions.
If, however, instead of initiating the connection from inside the image, you want to initiate the connection from (for example) the host to connect to something running on the qemu target (e.g. you want to ssh from your host to the target), then you'll need some more qemu networking magic. As I said, The Yocto Project's qemu-fu fully handles this scenario for you. All you need to do is supply a password for sudo!
However, there are a couple caveats. The networking that The Yocto Project sets up for you supplies a static IP to the image as it is being spun up. This is done by providing a kernel cmdline append providing the IP address to use. Providing a kernel cmdline append can only be done in qemu if the kernel lives outside the image. If you have created your own image which contains its kernel inside the root filesystem, you can't use this trick to assign an IP address.
This post and a number of follow-up posts titled "Qemu Networking Investigation" explore these issues. The first issue is figuring out how to get a qemu image to acquire an IP address without having to assign it statically inside the image itself when you can't assign it with a kernel cmdline append. The second issue is figuring out how to set up the networking so that you can reach services running inside the image.
Knowing these pieces of information are useful if you're using qemu to run, say, openSUSE or if you want to use The Yocto Project's *.vmdk option.