You can use Cosmos with QEMU, however we have removed specific support for QEMU because QEMU on Windows is horribly buggy and the developers being all Linux developers simply do not seem to care about the Windows distribution.
Several years ago a Windows specific bug existed and required only one line of code to change. Despite this the QEMU developers left the bug unfixed for several years, and in fact may still be unfixed.
Building QEMU on Windows manually has a lot of dependencies and is not a trivial task. One user even reported to us that the developers only test the Windows build against WINE on Linux, which has its own set of bugs and is not a good platform to test against as a substitute for a real installation of Windows.
This and other issues have caused us to simply give up on QEMU. We however have not and will not take any steps to block QEMU, but we will not make any steps to specifically support it again. If you do not want to use a virtualizer and need an emulator, we suggest Bochs instead.