rust: add clippy configuration file

Configure the minimum supported Rust version (though strictly speaking
that's redundant with Cargo.toml), and the list of CamelCase identifiers
that are not Rust types.

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

rust/clippy.toml

@@ -0,0 +1,2 @@
+doc-valid-idents = ["PrimeCell", ".."]
+msrv = "1.63.0"