Makefile: Don't install non-sphinx files in sphinx docs install

If we're doing an out-of-tree build of Sphinx, then we
copy some extra spurious files to the install directory
as part of 'make install':
qemu-ga-qapi.texi
qemu-ga-ref.7
qemu-ga-ref.7.pod
qemu-ga-ref.html
qemu-ga-ref.txt
qemu-qmp-qapi.texi
qemu-qmp-ref.7
qemu-qmp-ref.7.pod
qemu-qmp-ref.html
qemu-qmp-ref.txt

because these have been built into build/docs/interop along
with the Sphinx interop documents. Filter them out of the
set of files we install when we're installing the Sphinx-built
manual files. (They are installed into their correct locations
as part of the main install-doc target already.)

Fixes: 5f71eac0 ("Makefile, configure: Support building rST documentation")
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: 20190308135744.6480-4-peter.maydell@linaro.org