Move all crates to workspace root

[epilys]

Summary of the PR

Having all the workspace crates under the crates/ directory is unnecessary. Rust documentation itself recommends all crates to be in the root directory:

https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html#creating-the-second-package-in-the-workspace

I paste the text content here, in case the online page ever changes or becomes unavailable:

## Creating the Second Package in the Workspace

Next, let’s create another member package in the workspace and call it add_one. Change the top-level Cargo.toml to specify the add_one path in the members list:

Filename: Cargo.toml

[workspace]

members = [
    "adder",
    "add_one",
]

Then generate a new library crate named add_one:

$ cargo new add_one --lib
     Created library `add_one` package

Your add directory should now have these directories and files:

├── Cargo.lock
├── Cargo.toml
├── add_one
│   ├── Cargo.toml
│   └── src
│       └── lib.rs
├── adder
│   ├── Cargo.toml
│   └── src
│       └── main.rs
└── target

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[epilys]

Rebased against main.

[epilys]

Rebased against main.

[andreeaflorescu]

Looks like the pipeline also needs to be updated as the fuzz tests are now failing. Same for the coverage test.

[epilys]

Got to make sure CI works first...

[epilys]

@andreeaflorescu fixed the CI pipelines now, ready for review.

[stefano-garzarella]

Should we also update the following lines?

virtio-bindings/CONTRIBUTING.md:56:cp linux/v5_0_headers/*.rs vm-virtio/crates/virtio-bindings/src
virtio-bindings/CONTRIBUTING.md:57:mv vm-virtio/crates/virtio-bindings/src/virtio_net.rs vm-virtio/crates/virtio-bindings/src/virtio_net/generated.rs
virtio-queue/README.md:108:![queue](https://raw.githubusercontent.com/rust-vmm/vm-virtio/main/crates/virtio-queue/docs/images/queue.png)
virtio-queue/README.md:117:![descriptor](https://raw.githubusercontent.com/rust-vmm/vm-virtio/main/crates/virtio-queue/docs/images/descriptor.png)

[epilys]

@stefano-garzarella thanks, fixed! :)

[stefano-garzarella]

LGTM!

[epilys]

Rebased against main.

[epilys]

Ping

@andreeaflorescu @alexandruag @jiangliu @slp @stsquad