spice: require spice-protocol >= 0.8.1

spice: require spice-protocol >= 0.8.1

Requiring spice-server >= 0.8.2 is not enough since spice-server.pc
doesn't require spice-protocol (any version). Until that is fixed
upstream an explicit requirement in qemu fixes compilation broken since

commit 2e1a98c9c1b90ca093278c6b43244dc46604d7b7
Author: Alon Levy <alevy@redhat.com>
Date:   Fri Feb 24 23:19:30 2012 +0200

    qxl: introduce QXLCookie

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>