block: Use 'detect-zeroes' option for 'blockdev-change-medium'

Instead of modifying the new BDS after it has been opened, use the newly
supported 'detect-zeroes' option in bdrv_open_common() so that all
requirements are checked (detect-zeroes=unmap requires discard=unmap).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
diff --git a/blockdev.c b/blockdev.c
index 511260c..7b87bd8 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2546,6 +2546,7 @@
     BlockBackend *blk;
     BlockDriverState *medium_bs = NULL;
     int bdrv_flags;
+    bool detect_zeroes;
     int rc;
     QDict *options = NULL;
     Error *err = NULL;
@@ -2585,8 +2586,12 @@
         abort();
     }
 
+    options = qdict_new();
+    detect_zeroes = blk_get_detect_zeroes_from_root_state(blk);
+    qdict_put(options, "detect-zeroes",
+              qstring_from_str(detect_zeroes ? "on" : "off"));
+
     if (has_format) {
-        options = qdict_new();
         qdict_put(options, "driver", qstring_from_str(format));
     }
 
@@ -2623,8 +2628,6 @@
         goto fail;
     }
 
-    blk_apply_root_state(blk, medium_bs);
-
     qmp_blockdev_close_tray(has_device, device, has_id, id, errp);
 
 fail: