demarshaller: Fix palette marshalling
demarshaller: Fix palette marshalling

It turns out that using base + sizeof(struct) is not a good way to
access an array at the end of a struct. For SpicePalette sizeof is 16, but
offset of ents is 12. Using this calculation in the demarshaller breaks
things badly, so now we use the actual array member.

fgouget/spice-common,freedesktop-unofficial-mirror/spice__spice-common,Fantu/spice-common,SPICE/spice-common,elmarco/spice-common,elmarco/spice-common,fgouget/spice-common,Fantu/spice-common,freedesktop-unofficial-mirror/spice__spice-common,elmarco/spice-common,fgouget/spice-common,Fantu/spice-common,freedesktop-unofficial-mirror/spice__spice-common,elmarco/spice-common,SPICE/spice-common,fgouget/spice-common,Fantu/spice-common,SPICE/spice-common,freedesktop-unofficial-mirror/spice__spice-common