Hey Guix,

I saw QEMU was just updated to 2.12. I noticed a few things in the
release notes, namely:
* QEMU can now be built with Python 3.
* Support for SDL 1.2 is considered deprecated and will be removed
  entirely in a future release.

As a result, the first patch builds QEMU with Python 3 and SDL 2.

The second patch is optional and adds the GTK 3 frontend, which IMO
is a nice alternative to the SDL frontend.
This does increase the size of QEMU a bit. OTOH it seems unlikely that
most people who use QEMU don't already have GTK 3 installed in some
form.