rust: build: generate lint flags from Cargo.toml

rust: build: generate lint flags from Cargo.toml

Cargo.toml makes it possible to describe the desired lint level settings
in a nice format.  We can extend this to Meson-built crates, by teaching
rustc_args.py to fetch lint and --check-cfg arguments from Cargo.toml.
--check-cfg arguments come from the unexpected_cfgs lint as well as crate
features

Start with qemu-api, since it already has a [lints.rust] table and
an invocation of rustc_args.py.

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