spicec: only send display-config if the agent can handle it
spicec: only send display-config if the agent can handle it

elmarco/spice,Fantu/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,Fantu/spice,virtualAMP/spiceplay,ya-mouse/spice,Fantu/spice,SPICE/spice,fgouget/spice,elmarco/spice,virtualAMP/spiceplay,Fantu/spice,dezelin/spice,alon/spice-server,alon/spice-server,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,dezelin/spice,dezelin/spice,alon/spice-server,virtualAMP/spiceplay,mathslinux/spice,elmarco/spice,dezelin/spice,alon/spice-server,dezelin/spice,mathslinux/spice,ya-mouse/spice,fgouget/spice,ya-mouse/spice,freedesktop-unofficial-mirror/spice__spice,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,freedesktop-unofficial-mirror/spice__spice,SPICE/spice,mathslinux/spice,SPICE/spice,virtualAMP/spiceplay,mathslinux/spice,mathslinux/spice,fgouget/spice