block: Allow changing 'discard' on reopen

'discard' is one of the basic BlockdevOptions available for all
drivers, but it's not handled by bdrv_reopen_prepare() so any attempt
to change it results in an error:

   (qemu) qemu-io virtio0 "reopen -o discard=on"
   Cannot change the option 'discard'

Since there's no reason why we shouldn't allow changing it and the
implementation is simple let's just do it.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -3156,6 +3156,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
     BlockDriver *drv;
     QemuOpts *opts;
     QDict *orig_reopen_opts;
+    char *discard = NULL;
     bool read_only;
 
     assert(reopen_state != NULL);
@@ -3178,6 +3179,15 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
 
     update_flags_from_options(&reopen_state->flags, opts);
 
+    discard = qemu_opt_get_del(opts, "discard");
+    if (discard != NULL) {
+        if (bdrv_parse_discard_flags(discard, &reopen_state->flags) != 0) {
+            error_setg(errp, "Invalid discard option");
+            ret = -EINVAL;
+            goto error;
+        }
+    }
+
     /* All other options (including node-name and driver) must be unchanged.
      * Put them back into the QDict, so that they are checked at the end
      * of this function. */
@@ -3291,6 +3301,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
 error:
     qemu_opts_del(opts);
     qobject_unref(orig_reopen_opts);
+    g_free(discard);
     return ret;
 }