rust: add utility procedural macro crate

This commit adds a helper crate library, qemu-api-macros for derive (and
other procedural) macros to be used along qemu-api.

It needs to be a separate library because in Rust, procedural macros, or
macros that can generate arbitrary code, need to be special separate
compilation units.

Only one macro is introduced in this patch, #[derive(Object)]. It
generates a constructor to register a QOM TypeInfo on init and it must
be used on types that implement qemu_api::definitions::ObjectImpl trait.

Reviewed-by: Junjie Mao <junjie.mao@hotmail.com>
Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Link: https://lore.kernel.org/r/dd645642406a6dc2060c6f3f17db2bc77ed67b59.1727961605.git.manos.pitsidianakis@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/meson.build

@@ -14,6 +14,9 @@ _qemu_api_rs = static_library(
     '--cfg', 'MESON',
     # '--cfg', 'feature="allocator"',
   ],
+  dependencies: [
+    qemu_api_macros,
+  ],
 )
 
 qemu_api = declare_dependency(