/usr/share/applications/qemu.desktop should have an "Exec=" key.

According to the www.freedesktop.org .desktop-file specification, all "Application" desktop files should have an "Exec=" key. The one in qemu doesn't. 

This can be easily verified by running kbuildsycoca4 if KDE4 is present, but the issue is not DE-dependent.

Which binary exactly should be assigned as the default one, I don't know.