Makefile: Fix Sphinx documentation builds for in-tree builds

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 5f71eac06e15b9a3fa1134d446f 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: 5f71eac06e15b9a3fa1134d446f ("Makefile, configure: Support building rST documentation")
Reported-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 20190308135744[email protected]