docs/conf.py: Don't hard-code QEMU version

Don't hard-code the QEMU version number into conf.py. Instead
we either pass it to sphinx-build on the command line, or
(if doing a standalone Sphinx run in a readthedocs.org setup)
extract it from the VERSION file.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190305172139.32662-12-peter.maydell@linaro.org
Message-id: 20190228145624.24885-12-peter.maydell@linaro.org