Move all crates to workspace root

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

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>

Cargo.toml

@@ -2,6 +2,6 @@
 resolver = "2"
 
 members = [
-    "crates/vhost",
-    "crates/vhost-user-backend",
+    "vhost",
+    "vhost-user-backend",
 ]