migration: Disable multifd explicitly with compression

Multifd thread model does not work for compression, explicitly disable it.

Note that previuosly even we can enable both of them, nothing will go
wrong, because the compression code has higher priority so multifd feature
will just be ignored.  Now we'll fail even earlier at config time so the
user should be aware of the consequence better.

Note that there can be a slight chance of breaking existing users, but
let's assume they're not majority and not serious users, or they should
have found that multifd is not working already.

With that, we can safely drop the check in ram_save_target_page() for using
multifd, because when multifd=on then compression=off, then the removed
check on save_page_use_compression() will also always return false too.

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

migration/migration.c

@@ -1350,6 +1350,13 @@ static bool migrate_caps_check(bool *cap_list,
         }
     }
 
+    if (cap_list[MIGRATION_CAPABILITY_MULTIFD]) {
+        if (cap_list[MIGRATION_CAPABILITY_COMPRESS]) {
+            error_setg(errp, "Multifd is not compatible with compress");
+            return false;
+        }
+    }
+
     return true;
 }