docs: Fix installation of man pages with Sphinx 4.x

docs: Fix installation of man pages with Sphinx 4.x

The 4.x branch of Sphinx introduces a breaking change, as generated man
pages are now written to subdirectories corresponding to the manual
section they belong to. This results in `make install` erroring out when
attempting to install the man pages, because they are not where it
expects to find them.

This patch restores the behavior of Sphinx 3.x regarding man pages.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/256
Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>
Message-id: 20210503161422.15028-1-dgouttegattat@incenp.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>