If you'd like to forward X11 from a build machine to a machine running OSX, you'll need to install XQuartz.
Sadly the keyboard mapping will be incorrect. To solve this simply add these qemu params to your runqemu command:
runqemu qemuparams="-k en-us" qemux86
A couple of notes:
- keyboard mapping will be incorrect for all builds (core-image-minimal, core-image-sato) - for images without a GUI, you can also use the 'nographic' option: runqemu nographic qemux86 core-image-minimal