sheepdog: Use bdrv_coroutine_enter before BDRV_POLL_WHILE

When called from main thread, the coroutine should run in the context of
bs. Use bdrv_coroutine_enter to ensure that.
Signed-off-by: NFam Zheng <famz@redhat.com>

block/sheepdog.c

@@ -736,7 +736,7 @@ static int do_req(int sockfd, BlockDriverState *bs, SheepdogReq *hdr,
     } else {
         co = qemu_coroutine_create(do_co_req, &srco);
         if (bs) {
-            qemu_coroutine_enter(co);
+            bdrv_coroutine_enter(bs, co);
             BDRV_POLL_WHILE(bs, !srco.finished);
         } else {
             qemu_coroutine_enter(co);