Allow building accelerators as module.
Start with qtest as first user.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jose R. Ziviani <jziviani@suse.de>
Message-Id: <20210624103836.2382472-28-kraxel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>