rust: macros: check that #[derive(Object)] requires #[repr(C)]

rust: macros: check that #[derive(Object)] requires #[repr(C)]

Convert derive_object to the same pattern of first making a
Result<proc_macro2::TokenStream, CompileError>, and then doing
.unwrap_or_else(Into::into) to support checking the validity of
the input.  Add is_c_repr to check that all QOM structs include
a #[repr(C)] attribute.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>