build: add separate spice-protocol config option

When implementing spice vdagent protocol in qemu we only need the
spice-protocol package for that, spice-server is not needed.  So
go split those two build dependencies.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20210519053940.1888907-1-kraxel@redhat.com
Message-Id: <20210519053940.1888907-2-kraxel@redhat.com>