migration_protocol: use SPICE_MAGIC_CONST
migration_protocol: use SPICE_MAGIC_CONST

spice-protocol has a new define to create the magic constants, let's use
that.

ya-mouse/spice,fgouget/spice,ya-mouse/spice,SPICE/spice,fgouget/spice,fgouget/spice,SPICE/spice,ya-mouse/spice,SPICE/spice,fgouget/spice