Add ability to generate a drcov trace

Using the tracing facility would be cool if you can pass something like `AFL_QEMU_DRCOV_TRACE="/tmp/trace.drcov"` and qemu will trace the execution with `-trace exec_tb` and, once done, it convert the trace into drcov format like in https://github.com/RobertBuhren/qemutrace2drcov/blob/master/qemutrace2drcov.py that u can drop into lighthouse.

What u think?
