block: Use blk_co_preadv() for blk_read()

This patch introduces blk_co_preadv() as a central function on the
BlockBackend level that is supposed to handle all read requests from the
BB to its root BDS eventually.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/block/block_int.h

@@ -508,6 +508,10 @@ extern BlockDriver bdrv_qcow2;
  */
 void bdrv_setup_io_funcs(BlockDriver *bdrv);
 
+int coroutine_fn bdrv_co_do_preadv(BlockDriverState *bs,
+    int64_t offset, unsigned int bytes, QEMUIOVector *qiov,
+    BdrvRequestFlags flags);
+
 int get_tmp_filename(char *filename, int size);
 BlockDriver *bdrv_probe_all(const uint8_t *buf, int buf_size,
                             const char *filename);