rust: define prelude

Add a module that will contain frequently used traits and
occasionally structs.  They can be included quickly with
"use qemu_api::prelude::*".

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

rust/qemu-api/src/lib.rs

@@ -7,6 +7,11 @@
 #[rustfmt::skip]
 pub mod bindings;
 
+// preserve one-item-per-"use" syntax, it is clearer
+// for prelude-like modules
+#[rustfmt::skip]
+pub mod prelude;
+
 pub mod c_str;
 pub mod cell;
 pub mod definitions;