configure: Require at least spice-protocol-0.12.3

As of 5a49d3e9 we assume SPICE_PORT_EVENT_BREAK to be defined.
However, it is defined not in 0.12.2 what we require now, but in
0.12.3.  Therefore in order to prevent build failure we must
adjust our minimal requirements.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
