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

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 <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Acked-by: Aleksandar Markovic <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 20190305172139[email protected]
Message-id: 20190228145624[email protected]