client: exit nicely for --controller with no SPICE_XPI_SOCKET (rhbz#644292)
client: exit nicely for --controller with no SPICE_XPI_SOCKET (rhbz#644292)

When starting spicec with --controller, SPICE_XPI_SOCKET environment
variable must be defined so spicec and the controller can be connected.

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