qemu-img: Report bdrv_block_status failures

If bdrv_block_status_above() fails, we are aborting the convert
process but failing to print an error message.  Broken in commit
690c7301 (v2.4) when rewriting convert's logic.

Discovered when teaching nbdkit to support NBD_CMD_BLOCK_STATUS, and
accidentally violating the protocol by returning more than one extent
in spite of qemu asking for NBD_CMD_FLAG_REQ_ONE.  The qemu NBD code
should probably handle the server's non-compliance more gracefully
than failing with EINVAL, but qemu-img shouldn't be silently
squelching any block status failures. It doesn't help that qemu 3.1
masks the qemu-img bug with extra noise that the nbd code is dumping
to stderr (that noise was cleaned up in d8b4bad8).

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190323212639.579-2-eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

qemu-img.c

@@ -1630,6 +1630,8 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num)
                                           count, &count, NULL, NULL);
         }
         if (ret < 0) {
+            error_report("error while reading block status of sector %" PRId64
+                         ": %s", sector_num, strerror(-ret));
             return ret;
         }
         n = DIV_ROUND_UP(count, BDRV_SECTOR_SIZE);