gnu: Remove qemu-minimal-2.10.

* gnu/packages/virtualization.scm (qemu-minimal-2.10): Remove variable.
* gnu/packages/debug.scm (qemu-for-american-fuzzy-lop): New variable.
(american-fuzzy-lop)[inputs]: Remove custom-qemu and add
qemu-for-american-fuzzy-lop.
[arguments]: Adjust reference to QEMU in the 'install-qemu' phase.