block: merge qcow, vhdx and block_util into block crate

This commit merges crates `qcow`, `vhdx` and `block_util` into the
crate `block`, which can allow `qcow` to use functions from `block_util`
without introducing a circular crate dependency.

This commit is based on crosvm implementation:
https://chromium.googlesource.com/crosvm/crosvm/+/f2eecc4152eca8d395566cffa2c102ec090a152d

Signed-off-by: Yu Li <liyu.yukiteru@bytedance.com>

fuzz/fuzz_targets/block.rs

@@ -8,7 +8,7 @@
 
 #![no_main]
 
-use block_util::{async_io::DiskFile, raw_sync::RawFileDiskSync};
+use block::{async_io::DiskFile, raw_sync::RawFileDiskSync};
 use libfuzzer_sys::fuzz_target;
 use seccompiler::SeccompAction;
 use std::ffi;