qcow2-threads: split out generic path

Move generic part out of qcow2_co_do_compress, to reuse it for
encryption and rename things that would be shared with encryption path.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190506142741.41731-6-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/block/qcow2.h b/block/qcow2.h
index 8207ed3..637552e 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -350,8 +350,8 @@
     char *image_backing_format;
     char *image_data_file;
 
-    CoQueue compress_wait_queue;
-    int nb_compress_threads;
+    CoQueue thread_task_queue;
+    int nb_threads;
 
     BdrvChild *data_file;
 } BDRVQcow2State;