multifd: Make no compression operations into its own structure

It will be used later.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

---

No comp value needs to be zero.

migration/migration.h

@@ -300,6 +300,7 @@ bool migrate_auto_converge(void);
 bool migrate_use_multifd(void);
 bool migrate_pause_before_switchover(void);
 int migrate_multifd_channels(void);
+MultiFDCompression migrate_multifd_compression(void);
 
 int migrate_use_xbzrle(void);
 int64_t migrate_xbzrle_cache_size(void);