rust: qemu_api_macros: make pattern matching more readable and efficient

"let ... else" is useful when visiting syntax trees; it avoids multiple
levels of indentation and places the error close to the pattern.

While at it, use "ref" to avoid moving the syntax tree objects.

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