rust/qemu-api-macros: use syn::Error directly

rust/qemu-api-macros: use syn::Error directly

Our MacroError type wraps syn::Error as a variant, and uses another
variant for custom errors. Fortunately syn::Error can be used directly,
avoiding extra code on our side, so change the proc macro crate to use
it.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Link: https://lore.kernel.org/r/20250703-rust_macros-v1-1-b99f82febbbf@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>