fuzz: Add fuzzing infrastructure and QCOW fuzzer

Add the basic infrastructure for fuzzing along with a qcow fuzzer ported
from crosvm and adapted to our code.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

docs/fuzzing.md

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+# Fuzzing in Cloud Hypervisor
+
+Cloud Hypervisor uses [cargo-fuzz](https://github.com/rust-fuzz/cargo-fuzz) for fuzzing individual components.
+
+The fuzzers are are in the `fuzz/fuzz_targets` directory
+
+## Preparation
+
+Switch to nightly: 
+
+````
+rustup override set nightly
+````
+
+Install `cargo fuzz`: 
+
+```
+cargo install cargo-fuzz
+```
+
+## Running the fuzzers
+
+e.g. To run the `qcow` fuzzer using all available CPUs:
+
+```
+cargo fuzz run qcow -j `nproc`
+```
+
+## Adding a new fuzzer
+
+```
+cargo fuzz add <new_fuzzer>
+```
+
+Inspiration for fuzzers can be found in [crosvm](https://chromium.googlesource.com/chromiumos/platform/crosvm/+/refs/heads/master/fuzz/)