marshaller: Correctly determine if switches are fixed size
marshaller: Correctly determine if switches are fixed size

Switches are fixed size only if all cases have the same size *and*
it has a default case or all the valid cases are listed.

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