build: decouple Cargo.toml: split crate and workspace definitions

TL;DR: cargo clippy|check|... now runs on whole workspace by default.

## Steps

- add new workspace member `cloud-hypervisor`
- move `./src` to new workspace member
- move `./tests` to new workspace member
- move relevant parts from Cargo.toml to new workspace member
- kept necessary parts in main Cargo.toml, such as profile
  configurations

## About

The main Cargo.toml historically mixes workspace and crate definitions
for cloud-hypervisor and ch-remote. This makes it hard to read and
requires `--workspace` to run cargo clippy or cargo test on all
workspace members, which is counter-intuitive.

This patch separates the workspace from the crate definition in the main
Cargo.toml file. After this, cargo clippy, cargo test, etc., work on the
whole workspace naturally, giving a smoother developer experience. The
Cargo.toml without a package definition is also called a virtual
workspace or virtual manifest by Cargo [0].

Backporting is not a concern: CHV no longer backports, but the affected
files are rarely modified anyway.

[0] https://doc.rust-lang.org/cargo/reference/workspaces.html#virtual-workspace

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

tests/readme.md

@@ -0,0 +1 @@
+The integration tests have been moved to `./cloud-hypervisor/tests`.