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.

(cherry picked from spice-protocol commit 8a3def14e35039753097094ba356e85e8fc2d128)

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@redhat.com>

fgouget/spice-common,fgouget/spice-common,SPICE/spice-common,fgouget/spice-common,fgouget/spice-common,SPICE/spice-common,SPICE/spice-common