nixpkgs: afl - add QEMU support

This adds support for `afl-fuzz -Q`, which can be used to instrument
arbitrary black-box binary code for fuzz testing using American Fuzzy
Lop through QEMU emulation.

This requires a custom QEMU 2.2.0 build of the Linux userspace emulators
(system emulators aren't required) with some custom patches. Furthermore
we have to patch the patches a little to make the build more sane (there
are some notes in the README about this).

Overall, the addition of this feature by default doesn't significantly
impact build times (since building QEMU for only one target builds only
a fraction of the source code, and many features are disabled), so it's
enabled by default.

Signed-off-by: Austin Seipp <aseipp@pobox.com>