all: improvements to snapshot mode

[dvyukov]

List of possible improvements for the snapshot fuzzing mode:

1. Make qemu snapshot restoration faster (currently takes >1 sec on large kernel).
2. Figure out why it finds mostly fs bugs (can it find as many unique bugs in other subsystems?).
3. Apply snapshot fuzzing for driver fuzzing (snapshot in the beginning of hardware probing procedure, though, unclear if the same can be achieved with driver loading/unloading).
4. Relax sandboxing in snapshot mode to get more coverage:

• we don't need to isolate host networking b/c we don't use it, and can test various host drivers (virtio, e1000, etc)
• don't disable SNAPSHOT_FREEZE, SNAPSHOT_POWER_OFF, sched_setattr, reboot, init_module, etc