Makefile: Fix Sphinx documentation builds for in-tree builds

The Sphinx build-sphinx tool does not permit building a manual
into the same directory as its source files. This meant that
commit 5f71eac0 broke QEMU in-source-tree
builds, which would fail with:
  Error: source directory and destination directory are same.

Fix this by making in-tree builds build the Sphinx manuals
into a subdirectory of docs/.

Fixes: 5f71eac0 ("Makefile, configure: Support building rST documentation")
Reported-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190308135744.6480-2-peter.maydell@linaro.org