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>

virtio-devices/src/vhost_user/blk.rs

@@ -9,7 +9,7 @@ use crate::thread_helper::spawn_virtio_thread;
 use crate::vhost_user::VhostUserCommon;
 use crate::{GuestMemoryMmap, GuestRegionMmap};
 use crate::{VirtioInterrupt, VIRTIO_F_IOMMU_PLATFORM};
-use block_util::VirtioBlockConfig;
+use block::VirtioBlockConfig;
 use seccompiler::SeccompAction;
 use std::mem;
 use std::result;