channel-base: remove bad check of SpiceMsgWaitForChannels validity
channel-base: remove bad check of SpiceMsgWaitForChannels validity

SpiceMsgWaitForChannels is not packed. Comparing the original
msg size to SpiceMsgWaitForChannels is wrong.

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