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:
f2eecc4152
Signed-off-by: Yu Li <liyu.yukiteru@bytedance.com>
