hw/net/e1000: Move macreg[] arrays to .rodata to save 1MiB of .data

hw/net/e1000: Move macreg[] arrays to .rodata to save 1MiB of .data

Each array consumes 256KiB of .data. As we do not reassign entries,
we can move it to the .rodata section, and save a total of 1MiB of
.data (size reported on x86_64 host).

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Dmitry Fleytman <dmitry.fleytman@gmail.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20200305010446.17029-3-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>