Update copyright date for user-facing copyright strings

Update copyright date for user-facing copyright strings

Update the copyright date to 2020 for the copyright strings which are
user-facing and represent overall copyright info for all of QEMU.

Reported-by: John Arbuckle <programmingkidx@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200316112006.19107-1-peter.maydell@linaro.org