server: don't reset the display channel when disconnecting all its clients ,FDBZ #43977
server: don't reset the display channel when disconnecting all its clients ,FDBZ #43977

The display channel was unnecessarily set to NULL when we disconnect all the clients
(on flush display commands timeout).
As a result, we recreated the display channel when a new client was connected.
The display channel was created with default red_channel.client_cbs, while its
correct client_cbs are the ones that are set by the red_dispatcher when it creates
the first display_channel.
This fix enforces a single creation of the display channel (per qxl), via the red_dispatcher.

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