Document more clearly that EmulatorHooks.post_exec get called at the end of the harness

[vringar]

Is your feature request related to a problem? Please describe.
I just wasted multiple days trying to figure out why my fuzzer, that I based on qemu_baremetal/low_level, reported incorrect values for a memory location that I read from my custom module in the post_exec step.

This was due to the fact that the snapshot had already been restored and the memory reset to its original value

LibAFL/fuzzers/full_system/qemu_baremetal/src/fuzzer_low_level.rs

Line 192 in be21fae

qemu.restore_fast_snapshot(snap);

Describe the solution you'd like
Maybe change the post_exec name to post_harness? Also to differentiate against observers?

Describe alternatives you've considered
Update the comment to state that the hooks run after the harness

Additional context

What would be the correct way to read a chunk of memory after this line has returned?

LibAFL/fuzzers/full_system/qemu_baremetal/src/fuzzer_low_level.rs

Line 160 in be21fae

match emulator.qemu().run() {

I can't do it in an observer, because observers need to be serializable, so I can't add a Qemu struct to it.
I'm currently storing the return value of emulator.qemu().run() in a local var, reading the memory and then match of the stored value but that feels very hacky.