rust: cargo: store desired warning levels in workspace Cargo.toml

rust: cargo: store desired warning levels in workspace Cargo.toml

An extra benefit of workspaces is that they allow to place lint level
settings in a single Cargo.toml; the settings are then inherited by
packages in the workspace.

Correspondingly, teach rustc_args.py to get the unexpected_cfgs
configuration from the workspace Cargo.toml.

Note that it is still possible to allow or deny warnings per crate or
module, via the #![] attribute syntax.  The rust/qemu-api/src/bindings.rs
file is an example.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>