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>

rust/qemu-api/Cargo.toml

@@ -23,6 +23,5 @@ version_check = "~0.9"
 default = []
 allocator = []
 
-[lints.rust]
-unexpected_cfgs = { level = "warn", check-cfg = ['cfg(MESON)', 'cfg(HAVE_GLIB_WITH_ALIGNED_ALLOC)',
-                                                 'cfg(has_offset_of)'] }
+[lints]
+workspace = true

rust/qemu-api/meson.build

@@ -1,5 +1,5 @@
 _qemu_api_cfg = run_command(rustc_args,
-  '--config-headers', config_host_h, '--features', '--lints', files('Cargo.toml'),
+  '--config-headers', config_host_h, '--features', files('Cargo.toml'),
   capture: true, check: true).stdout().strip().splitlines()
 
 # _qemu_api_cfg += ['--cfg', 'feature="allocator"']