Makefile: Install qemu-[qmp/ga]-ref.* into the directory "interop"

Makefile: Install qemu-[qmp/ga]-ref.* into the directory "interop"

We need install qemu-[qmp/ga]-ref.* files into the subdirectory of qemu docs: interop.

If we visit the following address and click the link to qemu-qmp-ref.html:
https://www.qemu.org/docs/master/interop/bitmaps.html#basic-qmp-usage

It will report following error:
"
Not Found
The requested URL /docs/master/interop/qemu-qmp-ref.html was not found on this server.
"

Signed-off-by: Liao Pingfang <liao.pingfang@zte.com.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <1591663670-47712-1-git-send-email-wang.yi59@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>