channel: try TLS only once
channel: try TLS only once

A broken server may reply to switch to TLS again and again. spice-gtk
should only try once.

elmarco/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,guodong/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,elmarco/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,elmarco/spice-gtk,SPICE/spice-gtk