memory: Rename memory_region_do_writeback -> memory_region_writeback

memory: Rename memory_region_do_writeback -> memory_region_writeback

We usually use '_do_' for internal functions. Rename
memory_region_do_writeback() as memory_region_writeback().

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20200508062456.23344-2-philmd@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>