proto: Use proper type for preferred_compression field
proto: Use proper type for preferred_compression field

No need to use an uint8 typed member, we can directly use the
appropriate enum8 type here, which makes everything more explicit.

SPICE/spice-protocol,vdiot/spice-protocol,fgouget/spice-protocol,fgouget/spice-protocol,fgouget/spice-protocol,vdiot/spice-protocol,vdiot/spice-protocol,SPICE/spice-protocol,SPICE/spice-protocol