include/block/block_int: Document protocol related functions

Clarify that:

- for protocols the brdv_file_open function is used instead
of bdrv_open;

- when protocol_name is set, a driver should expect
to be given only a filename and no other options.
Signed-off-by: NFabiano Rosas <farosas@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

include/block/block_int.h

@@ -126,6 +126,8 @@ struct BlockDriver {
 
     int (*bdrv_open)(BlockDriverState *bs, QDict *options, int flags,
                      Error **errp);
+
+    /* Protocol drivers should implement this instead of bdrv_open */
     int (*bdrv_file_open)(BlockDriverState *bs, QDict *options, int flags,
                           Error **errp);
     void (*bdrv_close)(BlockDriverState *bs);
@@ -251,6 +253,12 @@ struct BlockDriver {
      */
     int coroutine_fn (*bdrv_co_flush_to_os)(BlockDriverState *bs);
 
+    /*
+     * Drivers setting this field must be able to work with just a plain
+     * filename with '<protocol_name>:' as a prefix, and no other options.
+     * Options may be extracted from the filename by implementing
+     * bdrv_parse_filename.
+     */
     const char *protocol_name;
     int (*bdrv_truncate)(BlockDriverState *bs, int64_t offset,
                          PreallocMode prealloc, Error **errp);